210 Professor Hoffmann on the Geological 
claybeforeour eyes. Itisonlyina scattered and isolated manner 
that we find in them traces of chemical action in the siliceous 
nature of the uniting basis, or in the erystalline-granular de- 
posits of quartz. The originally chemical formation of a large 
portion of the mountain masses of limestone is more probable, 
because that rock is deposited at present in a similar way 
from solutions in carbonated waters ; nevertheless, very many 
of them also, such as the limestones rendered impure by clay, 
resemble so perfectly the strata deposited by mechanical 
agency, that we cannot avoid regarding them as formed in a 
similar manner. We thus find, therefore, that the deposits 
produced in a chemical way are reduced to comparatively a 
very small portion of the crust of our globe, and these can 
have contributed but little to the elevation of the tempera- 
ture of the ancient world; for, a more close investigation 
proves that these, as well as all other deposits, at least all the 
finer mechanical deposits, must have been formed very gra- 
dually, and during very long periods. The occurrence of re- 
mains of plants and animals in the strata of the crust of the globe 
affords incontrovertible evidence on thishead; thuswe generally — 
find in limestone rocks the shells and zoophytes in whole beds 
together, and in all periods of life next one another; we find 
also broods of shells in the cavities of coral reefs, pholades in 
the hollows bored by themselves, and all these contained in 
regular strata. Hence, undoubtedly, a very long period must 
have elapsed while these strata were in the course of being 
deposited, and while they enveloped throughout a long time 
the bodies of animals of many successive generations, which 
had existed in a state of undisturbed development. Thus, also, 
the rocks of the coal formation contain in certain strata, in the 
greatest abundance, the products of a vegetation contempo- 
raneous with themselves, in a full grown state; colossal, and 
partly upright trees, must thus have had to perfect their growth 
previous to the deposition of the clays in which we find their 
individual portions at present enveloped. Hence it appears, 
that, though we should admit in its fullest extent the che- 
mical mode of formation of rocky strata, yet the influence of 
the heat developed by deposits can have been of very little 
moment. 
