BY E. G. HOGG, M.A. 139 



the rock becomes divided into irregular aggregates of sand 

 and calcite." 



It seems to me that the sample of the Cluirh sandstone 

 needs much consideration before it can be regarded as a real 

 support to Mr. Oldhnm's views as to the flexibility of the 

 Kahana rock. The difference between the two cases is fairly 

 obvious ; in the Kaliana stone the quartz grains interlock, 

 and the matrix is partially removed ; in the Cluirli rock the 

 interlocking takes p'ace not between the c|uartz grains, but 

 between the facets of the crystals of calcite forming the 

 base. The two explanations have, however, an important 

 feature in common, viz., the occurrence of free cavities which 

 may ramify into the rock in all directions. 



In this context it may be well to mention the occur- 

 rence of flexibility in a rock which is not sandstone. Mr. G. 

 W. Card* in 1892, drew attention to a flexible limestone 

 of Permian age developed at Marsden, in the County of 

 Durham, and at a point south of Sunderland. The rock is very 

 finely laminated, is very soft and friable, and in general 

 appearance not unlike a fine-grained sandstone It apjjears 

 (according to Sedgwick) to have resulted chemically from 

 deposition in successive layers. 



From sections prepared for the microscope it would appear 

 that a low power reveals a large number of irregularly shajjed 

 empty spaces, in the main ranged linearly indirections parallel 

 to the bedding, but also occurring promiscuously through the 

 section. The material of the slide is mainly an aggregate of 

 grains of dolomite, with a very few grains of quartz and specks 

 of blue and brown material. Mica is very rarely present. 

 The larger grains of dolomite appear to be intergrown in 

 such a way that the convexity of one fits into the concavity of 

 another. As a cause of flexibility, Mr, Card suggests : In the 

 first place room for internal movement is provided for by the 

 abundance of empty spaces, and in the second the structure 

 revealed by high magnifying power suggests the possibility 

 that many of the grains are interlocked in such a manner as 

 to permit of a certain amount of movement upon one another. 

 Owing, however, to the small size of the grains, Mr. Card was 

 unable to demonstrate whether the grains actually possessed 

 such power of movement or not. 



Mr. Card's paper is of the greatest interest ; the rock it 

 describes differs much both in point of age, composition, and 

 mode of origin, from Mr. Oldham's flexible rocks ; again, the 

 reality of the interlocking structure is far from certain, while 

 the existence of cavities allowing free play of molecular 

 movement seems well established. 



*Geol. Mag. (3) IX., 189.;, pp. 117, etc. 



