INTRODUCTION. 
89 
same causes, and the relations of the two conditions of these strata are readily 
understood. The connexion between the metamorphic and the non-metamorphic 
regions, of the mountainous and non-mountainous regions, the gradual dying out 
of the one and the gradual disappearance of the other, coincide with the lines of 
the accumulation and the gradual thinning of the strata constituting the series. 
We see, moreover, why the mineral and chemical constitution of the strata oc¬ 
cupying these relations, although of the same geological age, should not correspond 
with each other. 
In treating of metamorphic rocks, Sir Charles Lyell suggests an explanation 
of the cause of the general absence of calcareous matter from strata of this 
character, as follows* : 
“ The metamorphic strata, why less calcareous than the fossiliferous. — It has been 
remarked that the quantity of calcareous matter in metamorphic strata, or indeed in the 
hypogene formations generally, is far less than in fossiliferous deposits, Thus the crystalline 
schists of the Grampians in Scotland, consisting of gneiss, mica-schist, hornblende-schist, 
and other rocks, many thousands of yards in thickness, contain an exceedingly small pro¬ 
portion of interstratified calcareous beds, although these have been the objects of careful 
search for economical purposes. Yet limestone is not wanting in the Grampians, and it is 
associated sometimes with gneiss, sometimes with mica-schist, and in other places with other 
members of the metamorphic series. But where limestone occurs abundantly, as at Carrara 
and in parts of the Alps, in connection with hypogene rocks, it usually forms one of the 
superior members of the crystalline group. 
The scarcity, then, of carbonate of lime in the plutonic and metamorphic rocks generally 
seems to be the result of some general cause. So long as the hypogene rocks were believed 
to have originated antecedently to the creation of organic beings, it was easy to impute the 
absence of lime to the non-existence of those mollusca and zoophytes by which shells and 
corals are secreted ; but when we ascribe the crystalline formations to plutonic action, it is 
natural to inquire whether this action itself may not tend to expel carbonic acid and lime 
from the materials which it reduces to fusion or semi-fusion. Although we cannot descend 
into the subterranean regions where volcanic heat is developed, we can observe in regions 
of spent volcanoes, such as Auvergne and Tuscany, hundreds of springs, both cold and 
thermal, flowing out from granite and other rocks, and having their waters plentifully 
charged with carbonate of lime. The quantity of calcareous matter which these springs 
transfer, in the course of ages, from the lower parts of the earth’s crust to the superior or 
newly formed parts of the same, must be considerable! X. 
* Manual of Elementary Geology, 5th edition : American edition, page 623. 
t See Principles of Geology, by the Author : Index, “ Calcareous springs.” 
% [It seems to mo extremely doubtful if this calcareous matter be derived from any very deep 
source. H.] 
[ Paleontology III.] 
12 
