CLASSIFICATION OP METAMORPHIC PROCESSES 405 



sence of recrj^stallization in the Lower Cretaceous arkose and shale of the 

 Pasaj^ten series in British Columbia. Yet those beds were formerly be- 

 neath younger Cretaceous sediments probably more than 8,000 meters 

 thick. A similar condition is reported for the basal beds of the Creta- 

 ceous geosynclinal of northern California, where the thickness of cover 

 was likewise colossal. 



Such examples do show the subordinate importance of load metamor- 

 phism in later geological time. To a somewhat smaller extent the Paleo- 

 zoic geosynclinals fail to exhibit recrystallization in their lower strata. 

 The pressure has been high, the water content considerable, the composi- 

 tion of the sediments appropriate, the deep burial of long duration : and 

 yet metamorphism has been partial or nil. 



The latest Precambrian (Beltian) strata, on the other hand, are largely 

 recrystallized at horizons which have never been buried deeper than those 

 reached by the non-crystalline Mesozoic and Paleozoic sediments just 

 mentioned. The pre-Beltian stratified formations, from bottom to top, 

 the world over, have been almost entirely recrystallized. 



From darkens (1916, page 30) calculation of the whole amoimt of rock 

 that has ever been decomposed by the weather it is easy to form a rough 

 idea of the total volume eroded in geological time. But a fraction of 

 this total can be assigned to Precambrian time. Of that fraction only a 

 part represents the covers that lay on the surfaces of unconformity be- 

 tween the later Precambrian series and the older complexes of crystalline 

 schists. The complexes were highly metamorphic before the ancient 

 denudation corresponding to each of the unconformities. The combined 

 areas of the known complexes form a vast total. Much greater is the 

 total area of similar Precambrian terranes, reasonably supposed to under- 

 lie the existing Paleozoic and younger sediments. It seems safe to hold 

 that the average cover on the comj)lexes at the time of their recrystalliza- 

 tion was far less than 5,000 meters in thickness. PTence, if that change 

 were induced by vertical stress, the conditions must have been quite dif- 

 ferent from those which have ruled since the beginning of the Paleozoic. 



Assuming a steeper tliermal gradient for the earth in the pre-Beltian 

 era, as well as load metamorphism under a moderate cover, the ancient 

 recrystallization of the Shuswap terrane has been explained. In some- 

 what less concrete form the idea is found in writings as old as Hutton's. 

 It has been lost to sight by too many of the modern advocates of dynamic 

 metamorphism. A speculation involving the conception of an earth orig- 

 inally very liot Jiear the surface is no more dangerous than the fashionable 

 explanation of all. or nearlv all. I'ogional metamorphism by orogenic 

 movements: ^[ore probably than any other, the conception of load meta- 



