138 NOTE ON ITACOLUMITE OR FLEXIBLE SANDSTONE. 



difficulty is very hard to formulate. Mr. Oldham, it is only 

 fair to say, attributes much weight to the removal of the 

 matrix iu a suitable proportion. I cannot do better than 

 again quote from his paper. " The development of a flexible 

 structure depends on the propoi-tion and mode of distri- 

 bution of the felspathic mud ; if absent or only present in very 

 small proportion, decomposition will not extend deep into the 

 rock, the quartz grains will be detached and fall off, leaving 

 the un decomposed rock with a mere film of weathered stuff 

 on the surface ; if it is too evenly distributed, the quartz 

 grains will not be in sufficiently intimate contact with each 

 other, and as the rock weathers it will decompose into grains 

 of sand easily detached and removed ; if finally it should be 

 suitably distributed, but too large in amount, the voids left 

 by its removal will be so large that the quartz aggregates will 

 not interlock with each other." 



Mr. Oldham goes on to state, " the number of conditions 

 ■which must be fulfilled saiisfactorily accounts for the rarity of 

 flexible sandstone, and to a certain extent for the capriciousness 

 of its distribution in roclcs which are of the same nge and 

 have, to all appearance, the same composition and structure." 



With this statement of Mr. Oldham I am quite in accord ; 

 the removal of the matrix in just a suitable proportion 

 seems necessarv. With regard to the isotropic matrix, it 

 would appear that we have to look to a double metamorphism. 

 The rock was, we will assume, a normal sandstone initially ; 

 intense heat may have led to a partial fusion Avhereby the ex- 

 ternal surfaces of the quartz grains may have been trans- 

 formed into a glassy material ; at a later date solvent 

 action may have removed this matrix in such suitable pro- 

 portion as to give flexibility without disintegration. This is, 

 of course, mere hypothesis, but the importance of explaining 

 the isotropic base of the rock is at least as serious as the inter- 

 locking structure of the c|uartz. 



Mr. Oldham, in support of his view, attaches much im- 

 portance to the appearances presented by the flexible sandstone 

 near Charli, south of the Pemganga River in Berar. He states 

 " it is air ordinary soft sandstone of rounded grains of quartz 

 with a little felspar, held together by a cement of carbonate of 

 lime, which forms 35"9 per cent, of the whole mass. Here 

 there is no comparatively soluble material whose removal 

 leaves the rest of the rock as a mass of irregular aggregates 

 interlocking with each other, for on removal of the cement by 

 solution, the rock falls into sand. But if the fractured 

 surface of the rock is examined, an abundance of sheeny 

 patches point to a -crystallisation of the cementing matrix, and 

 these planes afford a number of planes along which solution 

 proceeds with greater rapidity than elsewhere, and as a result 



