GYPSUM. 49 



occur among the basal beds of the nummulitic series. They are not, 

 however, constant, and one was traced till it thinned out between the 

 adjacent beds of limestone. 



Thickness. — The disturbed state of the ground places much 

 difficulty in the way of estimating the thickness of what we may call 

 the gypseous series. In one section at Bahadur Khe'l, the gypsum has 

 no greater thickness than 32 to 40 feet, and the gray clays are there but 

 poorly represented. In other parts of the district one is perhaps inclined 

 to overestimate its depth from the manner of its exposure, presence of 

 superficial gypsum, &c, but in scyne cases it certainly seems to have a 

 thickness of, or approaching to, 200 feet, while with the clays, the whole 

 may average more than 300 feet. * 



Conditions of formation.— The circumstances accompanying the 

 deposition of this series as indicated by the rocks must have differed 

 considerably from those which obtained during the formation of the 

 underlying salt. The clearer waters were now liable to influx of clay or 

 mud, and again probably still and clear while the gypsum was being 

 deposited, becoming occasionally shallow and subject to currents which 

 left their traces, as they do now, in the form of ripple marks on sandy 

 or muddy bottoms. 



The amount of gypsum which water may dissolve is very small, 

 not being more than 2*5 of gypsum in 1,000 parts of water, and its 

 accumulation from aqueous solution in the present case would point 

 to long intervals during which earthy impurities were not present 

 in the water in any quantity, alternating in places with an opposite 

 state of things when mud or clay was greatly in excess. The deposition 

 of the gypsum from solution would doubtless have been hastened by 

 accession of heat, if this could be shown to have taken place, but whence 

 it might have been derived there is nothing to prove. The water may 

 have been at times so shallow as to have its temperature raised by 

 the heat of the sun, or thermal gypseous springs may have been in 

 operation (as shown to have been the case in the Spiti valley by 



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