238 
NATURE 
[ Fan. 11, 1883 
botany, and chemistry were not so widely diffused as now, 
it appeared to most writers necessary to devote some 
space, either in a prefatory or in an incidental way, to 
these sciences. Mineralogy and palzontology, growing 
with the growth of geology, were likewise treated with it. 
But owing to the rapid development of geology, its own 
subject matter has now become so voluminous that it can 
only with difficulty be outlined in the compass of a text- 
book, and step by step it has displaced everything of 
which a sufficient knowledge could be assumed. 
While mineralogy and paleontology have by their growth 
become more and more differentiated from geology, 
astronomy has been affiliated in a degree that was not 
anticipated. Previous to the revelations of the spectro- 
scope, our earth was regarded indeed in origin, composi- 
tion, and career as analogous to other planets, but only in 
a hypothetic and speculative way ; but now that there is 
a large body of evidence pointing to identity of composi- 
tion throughout the solar system, there is no longer any 
question of a common history, and every advance in 
celestial physics is now regarded as a contribution to the 
early history of the earth. A department of astronomical 
geology has thus arisen. 
In the work under consideration no space whatever is 
permitted to zoology and botany; chemistry is barely 
mentioned ; mineralogy (chiefly descriptive) is accorded 
only 25 pages ; paleontology proper is omitted, but 28 
pages are devoted to the principles of palzontological 
geology—a department of science clearly distinguishable 
if not distinct from paleontology, and inseparable from 
stratigraphy ; mythological cosmogony is not even men- 
tioned, but the space it has too often occupied is given to 
physiographical geology—a discussion of the origin of the 
physical features of the land. Astronomical geology is 
accorded 23 pages. The bulk of the volume—57o pages 
out of gto—is devoted to geognosy, and dynamical and 
structural geology that is, to rocks and rock structures, | 
and to the physical changes whereby rocks originate. 
Stratigraphy, which until very recently has arrogated the | 
lion’s share of space, is here reduced to less than one-third 
of the total. 
The distribution of space thus outlined is eminently 
judicious, and it may be doubted whether any could be | 
better adapted to the present status of the science and 
the present demands of instruction. If it has a fault it is 
in the amount it concedes to the demands of the geologist 
in the matter of stratigraphy. The student’s text-book 
has not yet been clearly differentiated from the geologist’s 
handbook, and there is certainly an open field to-day for 
a manual specially adapted to the use of the working 
geologist, and not primarily arranged for instruction, 
All of the larger text-books have been partially adjusted 
to this need, and Prof. Geikie’s is not an exception ; but 
in his work the adjustment appears only in the strati- 
graphical chapter, which embodies a mass of detail that 
can serve only to bewilder if the student undertakes to 
master it. If the 275 pages of descriptive stratigraphy 
were reduced to 50, and a portion of the space thus saved 
were devoted to a rapid review of the salient points of 
the geological history of some limited region, as Great 
Britain, for example, I am prone to believe that the 
student would be afforded a better insight into the aims 
and results of geological inquiry. 
In the classification and arrangement of the subject- 
matter of geological text-books, there has been as marked 
a development as in the scope. The number of different 
manners in which a congeries of allied topics can be 
grouped is practically limitless, for the bases of possible 
groupings are as numerous as the relations sustained by 
the topics ; but not all classifications are of equal utility, 
and at each stage in the progress of a science there is 
usually some one which commends itself as of superior 
advantage. As, in the progress of knowledge, new rela- 
tions are discovered, and the importance of relations 
previously known comes to be differently estimated, new 
classifications are adopted, in comparison with which the 
old appear crude. Geology is so young a science, that a 
single generation has witnessed a complete revolution in 
this regard. The primary classifications of the modern 
text-books have nothing in common with the earlier 
editions of Lyell’s manual. In the division and arrange- 
ment adopted by Geikie, only a single feature is original, 
but the order of presentation as a whole is new. 
The theme of geology is the history of the earth. In 
its study there are two lines of inquiry, which are so 
nearly independent that they form co-ordinate branches 
of the general theme: the one is cosmic, the other 
terrestrial. 
Cosmically considered, the earth is one ot a group of 
worlds believed to have a common origin, and to be pur- 
suing parallel courses of development, in which they have 
reached various different stages. Assuming this to be 
true, the less developed worlds present phases, through 
which the earth has already passed, and by studying 
them we may learn something of the youth of our planet. 
The terrestrial branch of inquiry is concerned chiefly 
with rocks. The changes of the crust have led to the 
formation of rocks, and have given to them great variety 
of composition and structure. It is known, moreover, 
that rock formation is still in progress, and that agencies 
whose operations can be witnessed are now forming many 
varieties of rocks, and are initiating many peculiarities of 
rock structure. It is possible, therefore, to associate 
certain rocks and rock structures with certain processes 
of change, and by this means to derive from a study of 
the rocks of the crust a history of the changes which led 
to their formation. This inquiry is greatly facilitated by 
the fact that rocks have been partly formed from animal 
and vegetable remains, and by the additional fact that 
there has been a progressive development of life; so that, 
the key once obtained, the chronological order of rocks can 
be deduced from their organic contents. 
In presenting the second line of inquiry as to the earth’s 
history it is therefore proper to treat: of the composition 
of rocks and other materials of the earth’s crust (geognosy) ; 
of.the forms in which rocks are aggregated, or the struc- 
ture of rock masses (structural geology) ; of the agencies 
which in modern times are observed to produce changes 
of the earth’s crust (dynamical geology) ; of the relation of 
organic remains to geological formations (palzontological 
geology) ; and finally, of the actual order in which the 
various kinds and groups of rocks succeed each other, 
and the deduced series of changes the earth has undergone 
(stratigraphical or historical geology). The first and last 
of these categories claim their respective positions without 
question: geognosy constitutes the alphabet of the sub- 
