404 University of California Publications in Geology [Vol. 10 



glaciated rock floors are found underlying the formation in various 

 localities. Oldham and others have correlated the Dwyka with the 

 Australian tillite and with the Talchir group of India. 



In North America the recognition of the glacial origin of the basal 

 conglomerate of the Cobalt series marks an important advance in our 

 knowledge of geological history. To A. P. Coleman 24 is due the credit 

 of this important discovery. "Wherever exposed the lower part of the 

 Cobalt series is seen to be a wonderfully thick, heterogeneous con- 

 glomerate. Hore 25 gives a good description of the rock. It consists 

 of a great variety of types and sizes of boulders with very little sorting 

 in evidence. Large, isolated boulders sometimes occur in clay slates ; 

 the larger masses are usually rounded or subangular, some are 

 markedly angular. The matrix is ordinarily graywacke with an 

 occasional slate pocket; numerous particles of quartz, feldspar, chert 

 and felsite are to be seen under the microscope. Some fine-grained 

 turbid matter in the matrix contains chlorite, thus differing from the 

 Dwyka. Within the deposit, sudden lateral and vertical changes often 

 occur ; shales are frequently interbedded with coarse material ; quartz- 

 ites, rapidly grading into akrose, interdigitate with the coarser facies. 

 Coleman 26 describes and figures well-preserved striations on some of 

 the boulders ; they show best on f elsites and fine-grained greenstones ; 

 many of the surfaces are well polished. In the Cobalt tillite it has 

 been possible to differentiate various types of glacial debris ; thus 

 widely separated fragments in a medium-grained matrix indicate 

 ground moraine ; wholly coarse material is considered as representing 

 the terminal moraine, while various aggregates of rudely sorted, 

 rounded pebbles mark the sites of ancient kames. In arriving at a 

 conclusion as to the origin of the Cobalt conglomerate, Coleman con- 

 sidered and rejected hypotheses involving crushing, faulting, talus 

 formation, and exceptionally heavy -river currents. The tillite is inter- 

 bedded with slate layers up to five hundred feet in thickness, which are 

 considered to represent deposition during interglacial periods. The 

 formation stretches over thousands of square miles and thus probably 

 indicates the extension of a great ice-sheet over the continent in that 

 remote time. 



Other ancient tillites in America have been found by Sayles 27 in 



-* Coleman, A. P., Jour. Geol., vol. 16, pp. 149-158, 1908. 

 as Hore, E, E., Jour. Geol., vol. 18, no. 5, pp. 459-67, 1910. 



26 Loc. ext. 



27 Sayles, E. W., Harv. Mus. Comp. Zool., vol. 56, no. 2, 1914. 



