XXIV INTRODUCTION, 



be.ratifull}' exhibited. The Sturgeon River area affords one of the best- 

 known ilkistrations of a schistose cong-lomerate the matrix of which has com- 

 pletely recrystallized and, therefore, can not be discriminated from a gneiss 

 of igneous origin, but contains numerous pebbles and bowldei-s flattened in 

 the plane of schistosity. 



The great Hemlock volcanic formation varies from rocks which are 

 altered chiefly by metasomatic change to those which have become com- 

 plete crystalline schists containing no vestige, either macroscopically or 

 microscopically, of a texture or structure which may. be interpreted as 

 igneous. 



SUBSBQUEKT HISTORY. 



After the introduction of the intrusives the region was subjected to vast 

 denudation, which reduced it approximately to its present configuration. 

 This period of erosion continued until late Cambrian time, when the sea 

 again overrode the district and deposited U23on the older rocks Upper Cam- 

 brian sediments. Long after the deposition of the Cambrian, and perhaps 

 later Paleozoic rocks, the district was again raised above the sea, and the 

 major part of the Cambrian deposits have been removed, although they are 

 found in patches throughout much of the district, and occur as a continu- 

 ous sheet just east of the area discussed. 



The district may have again been submerged in Cretaceous time ; but 

 if so, the deposits formed were removed after the area finally emerged from 

 the sea. Since Cretaceous time the region seems to have been one of 

 erosion. During the Pleistocene period a thick mantle of glacial deposits 

 was spread over the entire district. Since Pleistocene time erosion has 

 advanced far enough to uncover the rocks here and there. 



METAMORPHISM. 



The folding varied in its closeness in different parts of the district. 

 Moreover, the formations are of very variable character, including a great 

 variety of sediments and of igneous rocks. The formations, therefore, vary 

 greatly in their capacity to resist stresses. It thus follows that during the 

 folding process certain formations yielded to a much greater degree than 

 others. The amount of contained water and other conditions were also 

 variable. As a result of these many variable factors, it is one of the most 

 characteristic features of the district that there are tp be found nearly all 



