92 NEW YORK STATE MUSEUM 



brought out on the map by the lines of trend and the direction of 

 the numerous conglomerate, limestone and quartzite lenses, that are 

 carefully entered upon the map. The authors add that this struc- 

 ture can be traced north from the Bradshaw mountains to the 

 Jerome quadrangle, where the schists are invaded by granite 

 intrusions that show a similar northerly trend; and still farther 

 north is the Proterozoic Vischnu terrane of the Grand Canon, which 

 shows a north-northeast trend. Jaggar and Palache state that 

 the schists lie in isoclinal folds as the result of tight compression 

 by a horizontal force which acted from northwest to southeast. 

 This northeast-southwest trend is found as far east as the northwest 

 corner of Texas (Richardson, 19 14). 



From the cited publications which could be supplemented by 

 others, it follows that the general trend of the Precambrian folds in 

 the Colorado plateau is north with a tendency toward east, which leads 

 to variations to" north-northeast and northeast. This, however, 

 is the exact direction of the original Precambrian folding in the 

 Rocky mountains and the western part of the Canadian shield. 

 The most natural conclusion is then that the Precambrian folding 

 of the Colorado plateau is a direct continuation of that of the Rocky 

 mountain region and that both belong together as far as their Pre- 

 cambrian structure is concerned, in spite of the contrast between the 

 two as expressed in their present structure. This latter contrast 

 led Suess, as has been noted before, to suggest that the Colorado 

 plateau might be an outstanding southwestern block of Laurentia. 

 Our inference of the agreement of the Precambrian structure of 

 this plateau with that of Laurentia seems to corroborate this 

 suggestion. 



That the plateau of northern Mexico, which is but a southward 

 continuation of the Colorado plateau, would then also become linked 

 to Laurentia is an unavoidable corollary; the lack of any knowledge 

 of Precambrian rocks forbids, however, the discussion of this con- 

 clusion in this place. 



The Western plateau and Pacific coast regions have as yet not 

 afforded any conclusive evidence in regard to the trend lines of the 

 Precambrian folding for although there are large areas of meta- 

 morphic rocks of Precambrian aspect, their true age is in most 

 cases still quite uncertain and the presence of gneisses and mica 

 schists of Tertiary age is reported from the Coast range and lower 

 California. The presence of genuine Precambrian (Proterozoic) 

 rocks has, however, been reported in recent years from Oregon, 

 Idaho and California (Smith & Packard, 19 19, Knopf, 191 7 etc.), 



