Recent investigations upon the Laurentian rocks which have such an enormous development in Canada have shown that the views formerly held as to their structure and origin must to a large extent be modified. This change of views applies with equal force to the theories at one time put forth to account for the presence of certain economic minerals for which these rocks have long been celebrated.

Rock Groups op the Region.

Metamorphosed Sedimentaries.—From the composition, structural arrangement and general relations of large portions of that great rock system, it appears that certain beds found therein present many of the features peculiar to deposits originally sedimentary, and that these beds through extreme processes of metamorphism have been changed from their original condition of limestone, sandstone and possibly shale into marble, quartzite, slate and gneiss. Many of these beds are regularly interstratified, and this peculiarity can be readily seen . . .

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