ee Ss 
NALGRE 
261 
THURSDAY, JANUARY 18, 1883 
GEIKTE’S GEOLOGY} 
Geological Sketches at Home and Abroad. By Archibald 
Geikie, LL.D., F.R.S. With Illustrations. (London 
and New York: Macmillan and Co., 1882.) 
Text-Book of Geology. By Archibald Geikie, LL.D., 
F.R.S. With Illustrations. (London; Macmillan and 
Co., 1882.) 
Il. 
E now come to consider the quality of the matter 
contained in the volume, the discrimination exer- 
cised in its selection, the validity of the theories presented, 
and the fidelity with which the science is portrayed. It is 
the function of a text-book to exhibit to the student an im- 
partial and symmetric outline of the science. Its author 
is under obligation to present the views which are 
generally entertained by the great body of geologists, 
carefully withholding those which are peculiar to himself. 
From the great mass of available matter he must select 
that which will afford a well-balanced and comprehensive 
review, and he must sedulously avoid giving undue pro- 
minence to those matters which have special interest to 
himself by reason of his individual studies. In the work 
before us this has been accomplished in a manner which 
may truly excite admiration. Although the author is an 
original investigator in several departments of the science 
he delineates, he has permitted his own predilections to 
give little if any additional prominence to his special 
topics, and the wisdom he has displayed in the selection 
of material and the balancing of parts will commend itself 
to all professional readers. 
It is useless to attempt an analysis of a work which is 
itself an epitome of a great science, but we may refer to 
the treatment of a few mooted points and to a few matters 
of novelty or of current interest. 
The microscopical characters of rocks are treated more 
at length than in any other text-book. In the general 
account of rock characters they are accorded even more 
space than are the macroscopical, and they form part of 
the description of each specific rock. They are, more- 
over, illustrated by a series of cuts, showing the appear- 
ance of thin slices when highly magnified. A chapter is 
devoted to the subject of rock determination, and an 
analytical table is included therein. 
The results of the Ciad/enger exploration of the bottom 
of the ocean are given at some length, and the conclusion 
is drawn that the continental regions of the globe have 
been marked out from the earliest geological times. This 
is not treated as an hypothesis but as an established 
theory, and its logical consequences appear in numerous 
places. 
In the taxonomic terms of stratigraphy, the convention 
of the Bologna Congress is not adopted. The terms 
system, series, and stage are used in the same order, but 
group, which by the congress was made more compre- 
hensive than system, is by Geikie used as the equivalent 
of stage. He remarks, with propriety, that the attempt 
to alter the signification of a term so universally employed 
in English literature would produce far more confusion 
* Continued from p. 239. 
VOL. XXvII.—No. 690 
than can possibly arise from a failure to conform to 
continental usage. 
One of the most conspicuous omissions of the book is 
with reference to the antiquity of man. The subject is 
treated with great brevity, because it is regarded as 
belonging more properly to archzology, but an account 
is nevertheless given of the earliest human vestiges. 
Mention is made of man’s association with the Loess 
and with the inter-Glacial deposits of Europe, but the 
Californian claims to his pre-Glacial existence are ignored. 
It is true that these claims have been disputed, and it is 
true that the evidence in regard to each of the individual 
finds upon which they rest is incomplete; but since 
Whitney has assembled all the facts in his ‘ Auriferous 
Gravels,” it must be admitted that their cumulative force 
entitles them at least to recognition and consideration, 
however slow we may be to accept them as demonstrative. 
In the section which treats of manasa geological agent, 
there are enumerated a great variety of ways in which he 
modifies the face of nature, but one of the principal, if 
not indeed the chief of all, is omitted, namely, the stimu- 
lus he gives to denudation by tilling the soil. The mat 
of vegetation, living and decaying, which naturally covers 
the soil in all humid regions, affords great protection 
against the erosive work of rain. Not only is the beating 
of the rain resisted, but the velocity of its outflow is 
retarded, so that from surfaces of gentle inclination it 
washes away very few particles. When this mat has 
been removed, and especially when the surface has been 
stirred by the plough, the conditions become exceedingly 
favourable to rain erosion, and the rain rills are charged 
with sediment. Moreover, cultivation and the cutting of 
forests increase the magnitude of river floods, and since 
rivers perform their chief work of erosion and transporta- 
tion during flood-stage, the quantity of their work is thus 
augmented. It is safe to say that}the rate of degrada- 
tion of the surface by rains and rivers is increased several 
hundred per cent. by the removal of forests and the 
tillage of the soil, and it may be added that for this 
reason most attempts to measure the natural rate of 
denudation by means of the outscour of rivers have been 
abortive. 
The unconformability between the Archzean and the 
Palaeozoic is not mentioned in such way as to convey an 
impression of the profoundness of the chronological break. 
There is no known locality where a newer formation 
rests conformably upon the Archean. There are few 
where the discordance of dip is not great. There are 
few where the superior formation is not relatively unal- 
tered, and none where the inferior formation is not highly 
metamorphosed. So far as we know, the Archzean strata 
were both thrown in great folds and plicated in detail, 
were universally subjected to a metamorphism such as in 
later rocks seems to have been accomplished only at a 
depth beneath the surface, and were subsequently worn 
away upon a most stupendous scale before they received 
any sedimentary covering within the regions now acces- 
sible for examination. Compared with this, all other 
chronological breaks are trivial, and we may almost say 
that, compared with this, all other stratigraphical breaks 
are local. 
In treating of the condition of the interior of the earth, 
Geikie concisely presents the prominent hypotheses, and 
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