258 DR. A. GILLIGAN ON THE PETROGRAPHY OF [voJ. ixXV, 



Under the microscope the structure is even more remarkable, pre- 

 senting the appearance of a completely silicified oolite. The 

 concentric structure characteristic of oolites can be traced in 

 many cases. The matrix is crvptocrystalline and rnicrocrystallme 

 silica, and the oolitic grains are irregularly distributed through 

 the rock. No radial structure can be distinguished in any of 

 the oolitic grains. Each of the original oolitic grains is now 

 outlined by a number of small spherical bodies which are each 

 stained with oxide of iron. The diameter of these bodies is, on 

 the average (PL XVIII, fig. 5), - 02 to -03 mm. Similar rounded 

 bodies can be seen scattered through the ground-mass, and showing 

 a tendency to group together by twos and threes. In man}' of 

 these, when using a high magnification, can be seen a spherical 

 nucleus stained with haematite, while the rest of the body is quite 

 clear. The clear part has a somewhat higher refractive index than 

 the silica making up the ground-mass of the rock. The bodies 

 were at first suspected as being of organic origin, but the late 

 Dr. G. J. Hinde, who kindly examined the section for me, failed 

 to identify them as being derived from organisms. In the Geo- 

 logical Survey Memoir on the North- West Highlands of Scotland, 

 is described and figured a pebble from the Torridonian which 

 agrees in a most remarkable way with the one here described. 1 

 Silicified oolitic Durness Limestone kindly supplied to me by 

 Dr. John Home, F.R.S., has been examined for comparison, but 

 proved to be quite unlike this specimen, none of the curious 

 spherical bodies being found. The Rough Rock of Whitehall 

 Quarries, Horsforth, has also yielded a pebble with well-developed 

 oolitic structure, so that they have been found in the Lower, Middle, 

 and Upper Grit beds, but they are not of frequent occurrence by 

 any means. Another pebble from the Rough Rock of Horsforth 

 shows traces of organisms when a section is examined by ordinary 

 transmitted light (see PI. XVIII, fig, 6). They seem to be 

 tubular bodies which have been cut through in different directions. 

 Some of the tubes appear to be quite straight, while others are 

 curved. No structure can be observed even when a 1/9 objec- 

 tive is used. In polarized light almost all trace of the organisms 

 disappears, and the rock looks like a very fine-grained arenaceous 

 mudstone in which somewhat larger angular pieces of quartz are 

 irregularly distributed. Inside these angular pieces of quartz, 

 which is very clear, can be traced the circular sections of the 

 tubular organisms (by transmitted light), and this seems to point 

 to a secondary growth of silica round the organisms. 



A black siliceous pebble from the Kinderscout Grit of Embsay 

 Moor shows traces of organisms ; these appear to have been 

 brachiopods and sponge-spicules, while a very similar pebble 

 showing organisms was obtained from the grit of The Strid, 

 Bolton Abbey. 



1 'North- West Highlands of Scotland' Mem. Gaol. Surv. 1907. p. 280 & 

 pi. 1, fig. 1. 



