GEOLOGICAL SURVEY OF FORTIETH PARALLEL. 307 



the Wahsatch and Humboldt Mountains, from 12,000 to 14,000 feet, in 

 the Park and Medicine Bow Ranges somewhat less, and in the Clear 

 Creek region of Colorado at least 25,000 feet. This upper group will 

 be recognized by geologists as closely resembling the Huronian rocks 

 of the East. The Archaean nucleus of the Black Hills was reported 

 by the late Mr. Henry Newton to be composed of two groups of crys- 

 talline rocks closely resembling those described by Mr. King, and Mr. 

 George M. Dawson found a similar double series in Manitoba and 

 British Columbia. Without absolute proof which it would be dif- 

 ficult if not impossible to obtain the inference is at least allowable 

 that the rocks underlying the Palaeozoic series in the far West cor- 

 respond to the Laurentian and Huronian Groups of the Canadian ge- 

 ologists, and therefore that the foundation of the western half of the 

 continent is essentially the same with that of the eastern ; and also 

 that there, as here, a broad continental surface of these older rocks 

 supplied by erosion the mechanical material that entered into the com- 

 position of the Palaeozoic sediments, which, by successive oscillations 

 of sea-level, were spread to varying altitudes upon its flanks. 



At the close of his chapter on the Archaean, Mr. King proposes a 

 theory of the genesis of granite and crystalline schists, which is in 

 some respects new. In common with most of the geologists of the 

 present day, he supposes that the granites and schists are sedimentary 

 rocks which, having locally accumulated to great thickness, have sunk 

 by their own weight into the yielding crust of the earth to a point 

 where they have suffered more or less aqueo-igneous softening, and 

 then, in his view, under varying intensities of radial and tangential 

 pressure, they have been converted into corrugated schists or massive 

 granite, according to the less or greater energy of the forces acting 

 upon them. The evidence adduced by Mr. King to support this me- 

 chanical theory of the origin of granite is chiefly derived from the 

 facts which indicate internal and bodily movement in granite, such as 

 the dislocation of inclosed minerals, and the inclusion of masses of 

 foreign rocks. 



That there has sometimes been more movement in granites than in 

 the schists with which they are associated and of which they can fre- 

 quently be shown to be the exact equivalents in a more metamorphosed 

 condition is quite certain ; but it is very difficult to separate here the 

 effects of force from those of heat. Either produces the practical plas- 

 ticity which we see recorded in the obliteration of bedding, and the 

 inclusion of foreign rocks. Granites, which exhibit the extreme phase 

 of metamorphism, have evidently been in a plastic state, for they have 

 been forced into fissures of other rocks to form veins and extruded 

 mountain - crests proofs of softening and movement which schists 

 never afford but whether this plasticity was the effect of greater 

 heat or greater force than the associated schists suffered, is a question 

 not answered by any facts yet cited. The dislocation of included min- 



