. The Canadian field-naturalist. December, 19381 The Canadian Field-Naturalist 175. Folded Dolomite of Hastings Series, Lot 22, Concession X, Madoc Township, Hastings County, Ontario. If the Canadian Shield is a region of ancient mountains now truncated, then on its surface the interiors of these mountains are now ex- posed and from the rocks it should be pos- sible to learn something of what occurs in the interior of mountains when they are uplifted. An examination of the rocks of tbe Shield shows them to include a great many varieties, a considerable part of which are granite and 1 elated ig
. The Canadian field-naturalist. December, 19381 The Canadian Field-Naturalist 175. Folded Dolomite of Hastings Series, Lot 22, Concession X, Madoc Township, Hastings County, Ontario. If the Canadian Shield is a region of ancient mountains now truncated, then on its surface the interiors of these mountains are now ex- posed and from the rocks it should be pos- sible to learn something of what occurs in the interior of mountains when they are uplifted. An examination of the rocks of tbe Shield shows them to include a great many varieties, a considerable part of which are granite and 1 elated igneous types that have crystalized from a molten condition. The relationsthip of these igneous rocks show that they have been thrust into the suiperficial rocks with which they are associated, partly in masses both large and small, partly as injections along their planes of bedding, and partly as filling in fractures. It is evident, therefore, that as moun- ains are uplifted huge masses of molten rock are formed in their interiors which are thrust upward as an accompaniment of the crustal crumpling by which the mountains are formed. It is also known that the mountains of the Canadian Shield were not all uplifted at the same time but they were all worn down to approximately their present condition before the end of Pre-Cambrian time which occurred not less than several hixndred million years ago. The irregularity of the present surface of the Shield is partly the result of the varying resistance to weathering of the great variety of rocks composing it and partly the effect on denudation of the variable attitudes of its superficial rocks, both features resulting from the mountain building to which its rocks have been subjected. The peculiar physiographic characteristics of the Shield are therefore directly related to its former mountainous condition. The outcrops of crumpled strata shown in Figures 1 and 2 are found in the counties of Hastings and Peterborough, Ontario, near the s
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