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1 88 1.] Scepticism in Geology . 
rains down the sides of the fissure, denudation has slowly 
assisted expansion in producing the slopes which are charac- 
teristic of the valley. 
In some cases the expansive force has aCted unequally 
upon the sides of a crack in the crust of the Earth, and one 
side has been “ thrown up ” higher than the other. Hence 
huge masses of strata are sometimes exposed, whilst the 
corresponding side of the crack can with difficulty be disco- 
vered. In other cases, where a similar effeCt has been 
produced, the upheaved side, having apparently been simul- 
taneously with its slow elevation denuded by its exposure to 
air and water, cannot be distinguished on the surface. Up- 
heavals, like the latter, are common in the North of England, 
and are sources of great trouble and expense in connedtion 
with coal-mining operations. 
We have thus an explanation of the fadt unexpedtedly 
revealed by the Challenger Expedition, that the bottom of 
the deep ocean is not, as was commonly believed by geolo- 
gists, covered by the sediment carried down by rivers, but by 
accumulations of the shells of microscopic animals which 
have lived at or near the surface. Denudation has not taken 
place to the extent that was supposed, nor has all the matter 
thus carried down the sides of gorges and valleys found its 
way into the ocean. A large portion of it has served to fill- 
up constantly widening cracks and fissures, or to form the 
slowly extending alluvium of valleys consequent upon the 
Earth’s expansion. 
In order, however, to understand more perfedtly the 
changes which have taken place in the Earth’s surface and 
its present state, we must imagine its condition before the 
era of water and sedimentary deposits ; we must take lessons 
from the Moon and from our younger sister-planet, Venus. 
In an article which appeared in the “Journal of Science ” 
for January, 1877, Mr. Edmund Neison, F.R.A.S., observes 
that, “ many eminent physicists and geologists have recog- 
nised that the present condition of the surface of our satel- 
lite promises to throw much light on many vexed geological 
and physical problems. The opinion of that eminent geol- 
ogist, Prof. Phillips, on the subject is well known, for he has 
often expressed his conviction that the lunar surface pre- 
sented the best field for the study of many of the more 
difficult problems of Geology.” Mr. Neison further says 
that, “ It is a remarkable circumstance, in relation to the 
question, that whereas those astronomers who have devoted 
much time and labour to the study of the Moon’s surface, 
and to whom astronomers in general are mainly indebted for 
