EVALUATION OF STRATIGRAPIIIC CRITERIA 419 



nantly red, but some 10,000 feet of the middle part of the series are 

 characterized by flagstones and shales, with more or less calcareous 

 matter. Gray, green, and blue are prevailing colors in these, but no 

 marine fossil is known and mud cracks are remarkably developed. These 

 beds, therefore, represent the lower, poorly drained parts of an interior 

 fresh-water basin which was, however, dried out at repeated intervals. 

 The absence of saline deposits in these playa beds indicates that outflow 

 took place at times of high water, and the climate would seem, therefore, 

 to have been characterized by an alternation of dry and rainy seasons 

 rather than by a truly arid condition, such as has been conceived by cer- 

 tain British and Scotch geologists. This interpretation differs from 

 those commonly offered in that the basin is here thought to have con- 

 sisted of fluviatile plains and playa lakes rather than a great basin lake, 

 under whose waters all the sediments were deposited. 



Red shales and green sandstones — Subrecent example, the Siwalik for- 

 mation of India. — The second case of variegated beds consists of color 

 relations the reverse of those just considered. In place of deoxidized 

 shales and oxidized sandstones there are formations in which the shales 

 are oxidized and are interbedded with deoxidized argillaceous sandstones. 

 An example of such shales and sandstones which have accumulated under 

 conditions concerning which there is general agreement may be cited in 

 the Siwalik formations of the sub-Himalayas. These are Xeocene de- 

 posits upward of 15,000 feet in thickness skirting the southern side of 

 the Himalayas. They were laid down as fluviatile outwash from the ris- 

 ing mountains and have become exposed through being themselves u])- 

 turned and eroded in the latest movements. Medlicoll and Blanford 

 describe the Siwalik formations as follows :^^ 



"Sanflstoue iinniensely preponderates in the sul)-IIini:iljiyan deposits, and is 

 of a very persistent type from end to end of the rej^ion and from top to l)ottoni 

 of the series. Its commonest form is nndistint^uishahle from tlip roclv of cor- 

 responding? ajje known as Mohisso in the Alps, of a clear i)ci)pcr and salt irray. 

 sharp and fine in irrain, irencM-ally soft, and in very massive IxmIs. The whole 

 Middle and Lower Siwaliks are formed of this rock, with occasional thick 

 heds of red clay and very rare thin, discontinuous hands and nodules of earthy 

 limestone, the sandstone itself heins sometimes calcareous and thus cemented 

 into hard nodular masses. In the Sirnuir sroup (helow the Siwalik ^roup) 

 .generally, and locally in the Lower Siwaliks, the sandstone is thorousrhly 

 indurated and often of a purple tint, while retaininsc the distinctive aspect. 

 In the Upper Siwaliks conglomerates ])revail lar^rejy ; they are often made up 

 of the coarsest shingle, precisely like that in the heds of the great Himalayan 

 torrents. Brown cla.vs occnr often with the conglomerate, and sometimes 



i« Manual of the Geolog.v of India, part TI, lS7t>, pp. 524-520. 



