240 CHARLES KEYES 



eruptives. The slightly altered granites, diorites, and diabases 

 which cut the mass are manifestly relatively late intrusives. 



Their evident enormous extent, their intensely metamor- 

 phosed condition, their extensive deformation, their sharp lithologic 

 contrast with the superior metamorphics, and the marked erosional 

 unconformity dividing the two successions all attest the supreme 

 antiquity of the complex. The depositional equivalent of the 

 summital unconformity may itself surpass in duration the strati- 

 graphic record of the entire Paleozoic section. The best develop- 

 ments of the typical non-clastic Azoics are in the southwestern 

 portions of the state. 



The Archeozoic platform is bounded both above and below by 

 marked erosional planes of unconformity. The rocks are all 

 highly metamorphosed. The presence of quartzites, slates, and 

 marbles indicates the clastic origin of a large part of the mass. 

 The section is very thick, possibly not less than two miles. 



In marked contrast with the inferior Azoic rocks are the evi- 

 dences of a clastic origin of the major portion of the section, and 

 a distinct lithologic sequence is plainly discernible. Unconform- 

 ities which are associated may correspond to those shown in the 

 Grand Canyon, but it is believed that some of the latter are super- 

 posed in New Mexico. It is really the Archeozoic rocks chiefly 

 which heretofore have been called Archean, the Azoic masses not 

 being recognized and the Proterozoic segregation not being dif- 

 ferentiated. 



A thick sequence of pre-Cambrian elastics which are only 

 slightly altered is displayed in the Tijeras Canyon, in the Sandia 

 Range east of Albuquerque, where in a sharp fold a mile of strata 

 outcrops in continuous horizontal section. In discontinuous 

 exposure at least another, mile of beds is evidently present. The 

 strata are chiefly shales, locally more or less indurated with some 

 quartzite beds and intrusive granites. 



The quartzite beds which stand at high angles are commonly 

 mistaken for immense quartz reefs, and under such misconception 

 they are extensively prospected for gold. Microscopical examina- 

 tion in thin slices demonstrates conclusively that the rock has a 

 clastic origin, and that it is an old sandstone indurated by the 



