REPORT OF THE CHIEF ASTRONOMER 201 



SESSIONAL PAPER No. 25a 



How muck farther southward the geosyncline stretched is not easy to 

 declare, even in the tentative way held advisable for the extent of the geosyn- 

 cline as just outlined. There is something to be said for the correlation of the 

 Chuar series of Arizona with the lower part of the Belt-Cambrian group but in 

 that southern region the history of the geosyncline was complicated by orogenic 

 upturning, erosion, and subsidence, all of these affecting the strata older than 

 the Middle Cambrian Tonto sandstone. Just before Tonto time, therefore, 

 the geosynclinal sedimentation seems to have been, for a certain period, largely 

 or wholly interrupted in the latitude of northern Arizona. During the earlier 

 period we must believe, on the hypothesis that the Chuar series and Belt 

 terranes are, at least in part, stratigraphic equivalents, that the geosynclinal 

 extended still farther southward, perhaps into Mexico. 



In southwestern Colorado, Cross and his colleagues of the United States 

 Geological Survey have discovered a remarkable series of sections in ' Algon- 

 kian ' rocks, unconformably underlying the apparently Middle Cambrian 

 Ignacio quartzite of the region."" As now understood, the older rock series con- 

 sists of at least 8,000 feet of exceptionally massive quartzite with argillitic inter- 

 beds (the whole called the Uncompahgre formation), conformably overlying the 

 Yallecito conglomerate, 1,000 feet thick, which in turn rests on the Irving green- 

 stone, believed to be over 10,000 feet thick. /The relation of the greenstone and 

 conglomerate are obscure but an unconformity is postulated by the authors of the 

 reports on the Needle Mountains district. The evidence for this unconformity 

 largely consists in the fact that the conglomerate is composed of pebbles 

 derived from the greenstone. Neither that fact nor any other of those in favour 

 of the unconformity's existence can be regarded as showing a great period of 

 time as elapsing between the effusion of the lavas now represented in the Irving 

 greenstone, nor do the authors of the Needle Mountains folio state that there 

 has been any considerable time-gap at this horizon. 



The importance of the series in the present connection is that it seems to 

 correspond well with the basalt members of the Belt terrane as represented 

 in the Summit series" of the Selkirk mountains. The Irving greenstone is 

 certainly lithologieally very similar to the Irene volcanics of the Boundary 

 section, and it bears the same relation to the Briest River terrane as the Irving 

 greenstone bears to the Arehean schists of Colorado, except that the equivalent 

 of the Irene conglomerate is not directly apparent in the Colorado section. 

 The Yallecito conglomerate and Uncompahgre formation match well with the 

 Monk formation, as well as with the lower part of the massive Creston quart- 

 zite of the Burcell range. The question arises as to whether we have in this 

 Colorado section the southern part of the great Belt-terrane geosynclinal and, 

 in fact, the base of it near its thickest section. The relation of the Uncom- 

 pahgre formation to the Middle Cambrian Ignacio quartzite is like that of the 

 Chuar-Unkar series to the Middle Cambrian Tonto sandstone of the Grand 

 Canyon section. The correlation of all of these with the conformable series at 



* W. Cross, E. Ho.we, -J. D. Irving, and W. H. Emmons in the Needle Mountains 

 folio, 1905; and W. Cross, E. Howe and E. L. Ran&ome in the Silverton folio, 1905. 



