BY E. G. HOGG, M.A. 137 



to the decomposition and removal of this paste that the 

 flexibility of some specimens is due. In such a rock the 

 development of a flexible structure depends on the pro- 

 portion and mode of distribution of the felspathic mud." 



I have, I think, stated the essentials of Mr. Oldham's 

 theory, viz., the peculiar mode of aggregation of the quai'tz 

 grains, and the removal of a certain proportion of the 

 " felspathic mud" in which, to a more or less extent, the quartz 

 grains are included. The partial i-emoval of this enveloping 

 mud creates free spaces which the quartz grains may occupy 

 when stress is applied to the surface of the slab. 



A theory, apparently identical with that of Mr. Oldham, 

 was put forward in 1887 by Herr O. Miigge. j| 



Through the kindness of Mr. Morton, secretary of this 

 Society, I recently secured a small piece of flexible sandstone, 

 believed by Mr. Morton to have been l>rought from India. In 

 external appearance it does not differ appi'eciably from the 

 specimen exhibited by the Lord Bishop this evening. The 

 microscopic slides prepared show that the rock consists 

 mainly of quartz grains which had suffered little attrition 

 before deposition. Biotite and muscovite are both present, 

 but from their feeble developmentthey can hardly be regarded 

 as a main cause of the flexibility of the stone. In addition 

 the slide shows the occurrence, in fair quantity, of a matrix of 

 isotropic character containing much included matter. The 

 inclusions are, for the mostpai't, quartz grains of microscopic 

 dimensions and a small amount of opaque matter, the nature 

 of which I have failed to determine. This opaque matter is, 

 however, so subsidiary, as to suggestthat it does not play any 

 part in the explanation of the flexible nature of the rock. 



The slides appear to me to clearly show that part of 

 the paste originally enclosing the un-and sub-rounded grains 

 of quartz has been i-emoved. The slides do not throw 

 any light on the interlocking structure of the quartz on 

 which Mr. Oldham's theory largely depends. This negative 

 result is possibly due to the fact that my slides were not cut 

 in the direction I'equired to show up to advantage the inter- 

 locking structure, and the small piece of sandstone in my 

 possession did not admit of the preparation of many slides. 

 On consideration it does not appear clear that the " inter- 

 locking " of the quartz is the fundamental jjoint in any theory 

 brought forward to explain " flexible sandstone " It would 

 seem i-ather that a vera causa is to le found in the partial 

 removal of the matrix, whereby the quartz grains have free 

 play to move when the slab is stressed in any manner. As 

 regards the origin of an interlocking structure in the c^uartz 

 Mr. Oldham is silent, and indeed any theory to explain this 



II Neu. Jahrb. I Band., 1887, pp. 195-7. 



