PETROLOGY. 27 



A good deal of the material of the sand-rock was examined by 

 me, by means of washings and the microscope, in order to trace the 

 origin of the gold which is obtained in many of the rivers. The pure 

 kind was shown to be a fine-grained, pure sand, with a small amount 

 of argillaceous material. Clear quartzes, in slightly-rounded grains, 

 formed the bulk of the rock ; white mica, in flakes, was very com- 

 mon ; amethystine quartz was also numerous, together with some 

 deeper red coloured fragments of cornelian and jasper ; and a fair 

 percentage of iron oxide (magnetite), in fine rounded grains, was al- 

 ways discernible. 



In washing the material by hand, the mud and the mica flakes are 

 the first to run away ; the quartz forms up next on the edge of the 

 plate ; and last of all there is left a black border of magnetite. If gold 

 had appeared in the washing, it would have been found next to the 

 border of magnetite ; for it is a similar black magnetite residue, 

 among which the precious metal is found by the washers of the Sona 

 N. Certain portions of the sand-rock are, in addition, rich in fels- 

 par in irregular grains, and black mica. The whole of the material of 

 the rock is then much less rounded by water action. It is the magne- 

 tite and black mica which, mingling with the other constituents, give 

 the rock in some places the speckled, or pepper-and-salt, appearance. 



Taking the sand-rock stage altogether, we may say that the 

 upper portion of it is largely composed of the sandy and loamy beds. 

 The concretionary layers set in lower down ; and lower still the fine 

 quartzose conglomerates, and the clay conglomerates. The concre- 

 tionary sandy layers, and the conglomerates, continue down to the 

 base of the stage, and even pass into the upper beds of the Nahan 

 sandstone. 



The thickness of the sand-rock stage is very great ; and appears 

 sometimes to vary inversely as the thickness of the Siwalik conglo- 

 merate. For instance, there are 7,260 feet of the former, calculated 

 along the Ramganga, from the Nahan boundary up to the Siwalik 

 conglomerate ; whilst in the Kotri N. the thickness of the correspond- 

 ing band is 8,910 feet. It is true the northern boundary of the former 



( 85 ) 



