Invasion to Regional Met amor phism. 19 



depth at which took place the folding of the rocks now 

 exposed was certainly in places several miles, even allow- 

 ing for erosion accompanying folding. Chamberlin's 

 restoration, not allowing for erosion, shows a depth of 

 four and a half miles as a maximum on the west, and 

 six miles as a maximum on the east. In depth of burial 

 and intensity of deformation, this region must therefore 

 be regarded as having been subjected to conditions as 

 severe as those of western New England. No evidences 

 of Paleozoic igneous intrusion younger than the Cam- 

 brian are found in the region, the nearest of such intru- 

 sions being some pegmatite dikes in the Philadelphia 

 region. 



What, then, is the degree of regional metamorphism 

 as compared to western New England! It is found in 

 answer to be insignificant. The effects have been a re- 

 duction of porosity and a partial elimination of the com- 

 bined water from the argillaceous sediments. At the 

 base are limited thicknesses of Lower Cambrian quartzite 

 with some beds of hydromica slate. Above this, the 

 Cambro-Ordovician limestone shows no degree of altera- 

 tion. The upper Ordovician slates are dark and rather 

 dull in luster. Under the microscope they show a grain 

 beneath a tenth of a millimeter in diameter. Sericite 

 giving aggregate polarization is the most abundant 

 mineral, but kaolin also occurs.^ ^ There is thus seen to 

 be a profound difference from those coarse white marbles 

 and garnet-staurolite mica schists which occur in the 

 same structural belt in western New England. 



Since the depth and deformation have been great in 

 both cases, the difference can logically be ascribed only 

 to a lack in the Pennsylvania region of heat and crystal- 

 lizing solutions. This in turn suggests what is in accord 

 with the other lines of evidence, that magmas have under- 

 lain the one region at moderate depth, and have been 

 absent from the other. 



[Daly^^ reasons similarly from his work in British 

 Columbia, and cites authorities for other localities show- 

 ing that deep burial has produced metamorphism that is 

 only partial or nil.] 



(To be continued) 



^- T. N. Dale, U. S. Geol. Survey, BuU. 275, pp. 75-85, 1906. 

 " E. A. Daly, Bull. Geol. Soc. America, 28, 405, 1917. 



