592 
POPULAR SCIENCE REVIEW. 
partly water_, perhaps^ and partly ice, may seem due to tlie 
action of ice alone. It would not be safe to assert that ice 
may not in some cases have been the chief agent. But there 
are very many of similar form, and having identical physical 
characteristics, concerning which it may be said that their 
position *on the earth^s surface excludes this hypothesis. 
There are many districts where lakes abound, but where there 
is no proof, and indeed no probability, that ice has ever been 
present in large quantity. There are also lacustrine deposits 
among rocks where there is no evidence whatever of glacial 
action. The geologist and the physical geographer must care- 
fully observe where observation is possible, and infer where there 
is ground for inference. Thus, when we find that some of the 
principal lake basins of America are scooped out of horizontal 
strata, on low anticlinal and synclinal axes, it is clear that so 
far as the hollows have been cut they may be due to denudation 
and erosion either by water or ice. When in the Alps we find 
lake 'basins in valleys parallel or transverse to the main direc- 
tions of elevation, or along lines of fault, it is difficult to refuse 
belief that systematic fissures due to elevation have not had 
something to do with the phenomenon. Mountains are not 
indeed, in the sense and to the extent of the earlier geologists, 
the backbone of the earth, or the eternal and permanent frame- 
work upon and amongst which aqueous deposits have grown 
and attached themselves, like the fiesh of an animal on its 
skeleton. There is no real or useful analogy of this kiud. The 
rocks that form the nuclei of the great mountains are neither 
the oldest nor the most systematic ; they are often, no doubt, 
the hardest, and in that case they owe their position to their 
greater resisting power, when all around them was carried 
away, slowly and gradually, by weather and water. But it is 
just because this is the case, because the present form and 
outline is the result of a long past history, including periods of 
movement and periods of rest, but a never-ceasing wear and 
grind, that we find phenomena so varied in some respects, so 
distinct in others, but all due to the same group of causes 
acting continuously and only modified by local circumstances. 
The geologist and physical geographer must then accept, 
and have faith in, all natural causes, and he must not allow 
himself to attribute to any one an exclusive jurisdiction. He 
must admit and study the value of water and ice as real causes, 
but he must not deny that structure has in many, perhaps in 
all, cases, guided and governed the direction of the movement. 
He must be satisfied that lake basins, like other phenomena, 
are not to be accounted for by the assumption of one cause, 
but that they belong to the physical history of the globe, and 
result from those combinations that have also produced moun- 
tains, valleys, and plains. 
