262 
NATURE 
[ Fan. 18, 1883 
then remarks that it is “highly probable that the sub- 
stance of the earth's interior is at the melting-point 
proper for the pressure at each depth.” In treating of 
the age of the earth, he sets forth the geological and the 
physical arguments with commendable brevity, but with- 
holds all expression of individual opinion. In treating 
of the origin of orographical displacement he gives a brief 
history of opinion, and states that the contractional 
hypothesis is now generally accepted. A foot note, how- 
ever, refers to Fisher’s “ Physics of the Earth’s Crust,” 
which appeared while the text-book was passing through 
the press. The cause of ice motion is not discussed. 
In the classification of formations there is nothing new. 
The Cambrian and Silurian are marked as independent 
and co-ordinate divisions, the latter beginning with the 
Arenig group in Great Britain and with the Calciferous 
in America, but the opinion is expressed that a subse- 
quent revision of the subject may result in “ throwing all 
these older Palaeozoic rocks into one paleontological 
system.” The pre-Cambrian rocks are designated by 
Dana’s title of Archean. The Rheztic is included with 
the Trias. In the table of formations the American 
Laramie is placed in the Tertiary; but this appears to 
have been done by inadvertence, for in the descriptive 
text which follows it is treated as Cretaceous. 
In the classification of rocks the primary division is 
into crystalline and clastic. The crystalline are separated 
into stratified, foliated, and massive, and the clastic into 
sand rocks, clay rocks, volcanic fragmental, and organic 
fragmental. Of the massive crystalline rocks, the principal 
sub-group is indicated as fe/dspar bearing, and four small 
groups (the nepheline rocks, the leucite rocks, the olivine 
rocks, and the serpentine rocks) are indicated as co- 
ordinate with this. 
The subject of geological climate is treated almost ex- 
clusively from the astronomical point of view, and the 
theory of Croll is the only one which receives more than 
passing mention. Its statement was prepared especially 
for the volume by Dr. Croll himself, and covers six pages. 
It is undoubtedly true that this theory has been widely 
accepted, that it is very generally entertained as a working 
hypothesis, and that it is the most probable one before 
the public; and it should for these reasons be given great 
prominence in a text-book ; but I cannot help regretting 
that it has been presented with so little qualification. It 
deals with a series of physical laws and physical con- 
ditions which interact upon each other in an exceedingly 
complex way—in so complex a way that meteorologists, 
who have to deal with only a portion of them, do not 
claim and scarcely hope for a complete analysis of their 
combinations. The opportunities for arguing in a circle 
are most seductive, and the a fvvor2 probability that im- 
portant considerations have been overlooked is not small. 
The only manner in which so comprehensive and intricate 
an hypothesis can be established is by stimulating inquiry 
which shall lead to corroborative evidence, and this is 
precisely what Croll’s hypothesis after eight years of 
wide publicity has failed to do. If it is true, then epochs 
of cold must have occurred with considerable frequency 
through the entire period represented by the stratified 
rocks ; and iceberg drift, if no other traces, should have 
been entombed at numerous horizons. It has not been 
| 
Croll to show evidence of glacial action, the treatise 
under consideration mentions only two with confidence, 
and two oters with doubt. In the two instances to 
which queries are not attached, the phenomena appear to 
indicate local and not general glaciation. If the hypo- 
thesis is true, the cold of the Glacial epoch must have 
been many times interrupted by intervals of exceptional 
warm, but little has been added to the evidence adduced 
by Croll for such an interruption, and in America, where 
there is now great activity in the investigation of glacial 
phenomena, the evidence cf a szzg/e inter-glacial period 
is cumulative and overwhelming, while there is no indica- 
tion whatever of more than one. If the hypothesis is 
true, submergence in polar and temperate regions should 
have been coincident with glacial expansion, and emer- 
gence coincident with glacial retreat, but the Quaternary 
history of Great Britain, as drawn in the new text-book, 
includes two periods of maximum ice-extension, separated 
by a period of maximum submergence. While these 
difficulties exist it appears to me unadvisable to convey 
to the student the impression that a satisfactory solution 
to the problem of glacial climate has been reached. 
Because I have mentioned some points in which my 
individual judgment differs from that of. Prof. Geikie, it 
must not be supposed fora moment that I undervalue 
his work, or that I regard it with anything short of 
enthusiastic commendation. It is broad and catholic, 
conscientious in detail, masterly in treatment. It is 
imbued especially with a spirit which for want of a better 
name may be called scientific modesty. The majority of 
our text-books, including all of our best text-books, have 
been written by teachers, and have been more or less 
affected by the peculiar mental attitude of the teacher. 
The investigator is under the constant necessity of hold- 
ing his judgment in abeyance, and of treating every 
conclusion as an hypothesis, to be tested by future 
researches, and possibly amended or even abandoned. 
The teacher is under an equal necessity to formulate his 
knowledge so that he may communicate it in definite 
shape—he must not doubt, he must know; and under 
this compulsion he naturally and unconsciously acquires 
an undue confidence in results that have simply arisen 
from the weighing of probabilities. He is especially 
tempted to regard classifications as final, and not to recog- 
nise them as temporary presentations of temporary stages 
of knowledge. It is the especial merit of Prof. Geikie’s 
book that it is untainted by this teacher’s bias. It cautions 
the student against the confusion of geological synchrony 
with stratigraphical homotaxis; it cautions against the free 
use of palzontological evidence in the inference of geologi- 
cal climate; it cautions against deductions which may be 
vitiated by the imperfection of the geological record, and 
against negative evidence in general ; it cautions against 
the impression that there are in nature any hard and fast 
lines separating epochs or formations or rock species ; 
and, in addition, it heeds its own cautions. Its readers 
cannot escape the impression that the science of geology 
is in its youth, that it is developing at a rapid rate, that 
many of its results are tentative, and that its unsolved 
problems are as numerous and important as those it has 
successfully attacked. 
It is only by a conscious effort that one gives attention 
found, however, and of the eight horizons claimed by | to the literary style of Prof. Geikie’s text-book. It is so 
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