Fig. 23.—Lunar mountains near the Gulf of Iris.Fig. 23.—Lunar mountains near the Gulf of Iris.
The most curious feature on the moon's surface are the bands of lighter colour, which, radiating from certain of the volcanolike pits—those of lesser size and probably of latest origin—extend in some cases for five hundred miles or more across the surface. These light bands have never been adequately explained. It seems most likely that they are stains along the sides of cracks, such as are sometimes observed about volcanoes.
The eminent peculiarity of the moon is that it is destitute of any kind of gaseous or aqueous envelope. That there is no distinct atmosphere is clearly shown by theperfectly sharp and sudden way in which the light of a star disappears when it goes behind the moon and the clear lines of the edge of the satellite in a solar eclipse. The same evidence shows that there is no vapour of water; moreover, a careful search which the writer has made shows that the surface has none of those continuous down grades which mark the work of water flowing over the land. Nearly all of the surface consists of shallow or deep pits, such as could not have been formed by water action. We therefore have not only to conclude that the moon is waterless, but that it has been in this condition ever since the part that is turned toward us was shaped.
As the moon, except for the slight movement termed its "libration," always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold. There are reasons why this is improbable. In the first place, we see on the extreme border of the moon, when the libration turns one side the farthest around toward the earth, the edge of a number of the great walled pits such as are so plenty on the visible area; it is fair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side. Furthermore, there are certain bands of light which, though appearing on the visible side, evidently converge to some points on the other. It is reasonable to suppose that, as all other bands radiate from walled pits, these also start from such topographic features. In this way certain likenesses of the hidden area to that which is visible is established, thus making it probable that the whole surface of the satellite has the same character.
Clearly as the greater part of the moon is revealed to us—so clearly, indeed, that it is possible to map any elevation of its surface that attains the height of five hundred feet—the interpretation of its features in the lightof geology is a matter of very great difficulty. The main points seem to be tolerably clear; they are as follows: The surface of the moon as we see it is that which was formed when that body, passing from the state of fluidity from heat, formed a solid crust. The pits which we observe on its surface are the depressions which were formed as the mass gradually ceased to boil. The later formed of these openings are the smaller, as would be the case in such a slowing down of a boiling process.
As the diameter of the moon is only about one fourth of that of the earth, its bulk is only about one sixteenth of that of its planet; consequently, it must have cooled to the point of solidification ages before the larger sphere attained that state. It is probable that the same changeless face that we see looked down for millions of years on an earth which was still a seething, fiery mass. In a word, all that vast history which is traceable in the rocks beneath our feet—which is in progress in the seas and lands and is to endure for an inconceivable time to come—has been denied our satellite, for the reason that it had no air with which to entrap the solar heat and no water to apply the solar energy to evolutionary processes. The heat which comes upon the moon as large a share for each equal area as it comes upon the earth flies at once away from the airless surface, at most giving it a temporary warmth, but instituting no geological work unless it be a little movement from the expansion and contraction of the rocks. During the ages in which the moon has remained thus lifeless the earth, owing to its air and water, has applied a vast amount of solar energy to geological work in the development and redevelopment of its geological features and to the processes of organic life. We thus see the fundamental importance of the volatile envelopes of our sphere, how absolutely they have determined its history.
It would be interesting to consider the causes which led to the absence of air and water on the moon, but thismatter is one of the most debatable of all that relates to that sphere; we shall therefore have to content ourselves with the above brief statements as to the vast and far-acting effects which have arisen from the non-existence of those envelopes on our nearest neighbour of the heavens.
So far as possible the preceding pages, by the method adopted in the presentation of facts, will serve to show the student the ways in which he may best undertake to trace the order of events exhibited in the phenomena of the earth. Following the plan pursued, we shall now consider certain special points which need to be noted by those who would adopt the methods of the geologist.
At the outset of his studies it may be well for the inquirer to note the fact that familiarity with the world about him leads the man in all cases to a certain neglect and contempt of all the familiar presentations of Nature. We inevitably forget that those points of light in the firmament are vast suns, and we overlook the fact that the soil beneath our feet is not mere dirt, but a marvellous structure, more complicated in its processes than the chemist's laboratory, from which the sustenance of our own and all other lives is drawn. We feel our own bodies as dear but commonplace possessions, though we should understand them as inheritances from the inconceivable past, which have come to us through tens of thousands of different species and hundreds of millions of individual ancestors. We must overlook these things in our common life. If we could take them into account, each soul would carry the universe as an intellectual burden.
It is, however, well from time to time to contemplate the truth, and to force ourselves to see that all this apparently simple and ordinary medley of the world about us is a part of a vast procession of events, coming forth from the darkness of the past and moving on beyond thelight of the present day. Even in his professional work the naturalist of necessity falls into the commonplace way of regarding the facts with which he deals. If he be an astronomer, he catalogues the stars with little more sense of the immensities than the man who keeps a shop takes account of his wares. Nevertheless, the real profit of all learning is in the largeness of the understanding which it develops in man. The periods of growth in knowledge are those in which the mind, enriched by its store, enlarges its conception while it escapes from commonplace ways of thought. With this brief mention of what is by far the most important principle of guidance which the student can follow, we will turn to the questions of method that the student need follow in his ordinary work.
With almost all students a difficulty is encountered which hinders them in acquiring any large views as to the world about them. This is due to the fact that they can not make and retain in memory clear pictures of the things they see. They remember words rather than things—in fact, the training in language, which is so large a part of an education, tends ever to diminish the element of visual memory. The first task of the student who would become a naturalist is to take his knowledge from the thing, and to remember it by the mental picture of the thing. In all education in Nature, whether the student is guided by his own understanding or that of the teacher, a first and very continuous aim should be to enforce the habit of recalling very distinct images of all objects which it is desired to remember. To this end the student should practise himself by looking intently upon a landscape or any other object; then, turning away, he should try to recall what he has beheld. After a moment the impression by the sight should be repeated, and the study of the memory renewed. The writer knows by his own experience that even in middle-aged people, where it is hard to breed new habits, such deliberate training can greatly increase the capacity of the memory for taking in and reproducing images which are deemed of importance. Practice of this kind should form a part of every naturalist's daily routine. After a certain time, it need not be consciously done. The movements of thought and action will, indeed, become as automatic as those which the trained fencer makes with his foil.
Along with the habit of visualizing memories, and of storing them without the use of words, the student should undertake to enlarge his powers of conceiving spaces and directions as they exist in the field about him. Among savages and animals below the grade of man, this understanding of spacial relations is very clear and strong. It enables the primitive man to find his way through the trackless forest, and the carrier pigeon to recover his mate and dwelling place from the distance of hundreds of miles away. In civilized men, however, the habit of the home and street and the disuse of the ancient freedom has dulled, and in some instances almost destroyed, all sense of this shape of the external world. The best training to recover this precious capacity will now be set forth.
The student should begin by drawing a map on a true scale, however roughly the work may be done, of those features of the earth about him with which he is necessarily most familiar. The task may well be begun with his own dwelling or his schoolroom. Thence it may be extended so as to include the plan of the neighbouring streets or fields. At first, only directions and distances should be platted. After a time to these indications should be added on the map lines indicating in a general way contours or the lines formed by horizontal planes intersecting the area subject to delineation. After attaining certain rude skill in such work, the student may advantageously make excursions to districts which he can see only in a hurried way. As he goes, he should endeavour to note on a sketch map the positions of the hills and streams and the directions of the roads. A year of holiday practice in such work will, if the tasks occupy somewhereabout a hundred hours of his time, serve greatly to extend or reawaken what may be called the topographic sense, and enable him to place in terms of space the observations of Nature which he may make.
In his more detailed work the student should select some particular field for his inquiry. If he be specially interested in geologic phenomena, he will best begin by noting two classes of facts—those exhibited in the rocks as they actually appear in the state of repose as shown in the outcrops of his neighbourhood, and those shown in the active manifestations of geological work, the decay of the rocks and the transportation of their waste, or, if the conditions favour, the complicated phenomena of the seashores.
As soon as the student begins to observe, he should begin to make a record of his studies. To the novice in any science written, and particularly sketched, notes are of the utmost importance. These, whether in words or in drawings, should be made in face of the facts; they should, indeed, be set down at the close of an observation, though not until the observer feels that the object he is studying has yielded to him all which it can at that time give. It is well to remark that where a record is made at the outset of a study the student is apt to feel that he is in some way pledged to shape all he may see to fit that which he has first written. In his early experience as a teacher, the writer was accustomed to have students compare their work of observation and delineation with that done by trained men on the same ground. It now seems to him best for the beginner at first to avoid all such reference of his own work to that of others. So great is the need of developing independent motive that it is better at the outset to make many blunders than to secure accuracy by trust in a leader. The skilful teacher can give fitting words of caution which may help a student to find the true way, but any reference of his undertakings to masterpieces is sure to breed a servile habit. Therefore suchcomparisons are fitting only after the habit of free work has been well formed. The student who can afford the help of a master, or, better, the assistance of many, such as some of our universities offer, should by all means avail himself of this resource. More than any other science, geology, because of the complexity of the considerations with which it has to deal, depends upon methods of labour which are to a great extent traditional, and which can not, indeed, be well transmitted except in the personal way. In the distinctly limited sciences, such as mathematics, physics, or even those which deal with organic bodies, the methods of work can be so far set forth in printed directions that the student may to a great extent acquire sound ways of work without the help of a teacher.
Although there is a vast and important literature concerning geology, the greater part of it is of a very special nature, and will convey to the beginner no substantial information whatever. It is not until he has become familiar with the field with which he is enabled to deal in the actual way that he can transfer experience thus acquired to other grounds. Therefore beyond the pleasing views which he may obtain by reading certain general works on the science, the student should at the outset of his inquiry limit his work as far as possible to his field of practice, using a good text-book, such as Dana's Manual of Geology, as a source of suggestions as to the problems which his field may afford.
The main aim of the student in this, as in other branches of inquiry, is to gain practice in following out the natural series of actions. To the primitive man the phenomenal world presents itself as a mere phantasmagoria, a vast show in which the things seen are only related to each other by the fact that they come at once into view. The end of science is to divine the order of this host, and the ways in which it is marshalled in its onward movement and the ends to which its march appears to be directed. So far as the student observes well, and thus gains a clearnotion of separated facts, he is in a fair way to gather the data of knowledge which may be useful; but the real value of these discernments is not gained until the observations go together, so as to make something with a perspective. Until the store of separate facts is thus arranged, it is merely crude material for thought; it is not in the true meaning science, any more than a store of stone and mortar is architecture. When the student has developed an appetite for the appreciation of order and sources of energy in phenomena, he has passed his novitiate, and becomes one of that happy body of men who not only see what is perceived by the mass of their fellows, but are enabled to look through those chains of action which, when comprehended, serve to rationalize and ennoble all that the senses of man, aided by the instruments which he has devised, tell us concerning the visible world.