106 GEOLOGY OF THE SON VALLEY, ETC. 



strata of the Garbandh outlier, belonging to the division No. i of the 

 'ower Vindhyans, and containing a large proportion of " trappoids " 

 and "porcellanites." In the hand-specimen it has an irregular aspect 

 resembling the porcellanites at one place, while in other portions 

 the admixture of included fragments gives it the appearance of the 

 * ( trappoids," while lastly some parts are so quartzitic as to resem" 

 ble a tuffaceous sandstone, all these changes taking place in the space 

 of a few inches. Under the microscope the tuffaceous nature of 

 the rock seems evident from the extreme irregularity in size and 

 frequent angularity of the fragments. The presence of numerous 

 fragments of a rhyolitic groundmass shows moreover that generally the 

 component fragments are to a large extent volcanic. At the same 

 time, there are other fragments undoubtedly derived from the gneiss, 

 such as pieces of granulated mosaic quartz and fragments of tour- 

 maline. This rock therefore may be regarded as a passage type 

 between tuffaceous and true aqueous sediments. Secondary deposi- 

 tion of quartz, which is the cause of the compact nature of the porcella- 

 nites and trappoids, is rendered extremely conspicuous in this rock 

 and in the one next to be described owing to secondary growth of 

 the larger quartz-grains. 



-gW which occurs a short distance from the rock just described, 

 is a light-grey compact sandstone, so felspathic that under the influ- 

 ence of weathering it becomes covered with a decomposition crust 

 very similar to that which forms upon the " trappoids.' ' It also 

 possesses the sub-conchoidal fracture of these rocks. It consists of 

 quartz, orthoclase, and rhyolite fragments, but the microscopic 

 structure is quite different from that presented by the " trappoids/' 

 It is a true sandstone of regular structure in which all the grains are 

 of sensibly the same dimension. They are all well rounded and the 

 quartz is largely in excess of the grains of felspar and rhyolite. It 

 is probable that a large portion of the quartz and perhaps all the 

 felspar may be volcanic in origin, but the fragments have been com- 

 pletely redistributed by a process of true sedimentation. The com* 

 pact nature of the rock is due to considerable 



Secondary silicification. ... , n ^. m, . c t 



secondary silicification. The grains of quartz 

 ( 106 ) 



