HISTORY OP GRITS AND SANDSTONES. 



Sandstones, or rather quartzites, of Upper-Llandovery age are 

 developed in the Lower Lickey hills in Worcestershire. A specimen 

 from this locality was found to be chiefly composed of much-rounded 

 grains of quartz, having an average diameter of inch, cemented 

 together by a growth of transparent crystalline quartz. This mode 

 of formation is rendered evident by the circumstance of the rounded 

 grains being frequently composed of cloudy or slightly turbid quartz, 

 while the cementing silica is perfectly colourless and transparent. 

 When examined in polarized light, the cementing quartz is seen to 

 exhibit, for a certain, distance around each grain, the same colour as 

 the grain itself, and appears to have been deposited in crystalline 

 continuity therewith. Pluid- cavities, with bubbles, are numerous in 

 some of the grains, while others are entirely without them. Eutile 

 and schorl are sometimes present in the form of minute crystals. 



A specimen of Denbighshire grit from Pont Cietwr Yspytty, 

 when examined, was found to consist of an exceedingly fine-grained 

 mosaic of cementing concrete containing minute granules of quartz, 

 and enclosing larger fragments of the same mineral, with felspar and 

 colourless or brown mica, each grain being from to y-—- inch in 

 diameter. Some of the quartz fragments are so traversed by the 

 moss-like greenish mineral, often forming a constituent of the cement 

 of sandstones, as almost to suggest the idea of their being pseudo- 

 morphs after a mineral which has disappeared. If seen in polarized 

 light, however, they will be observed to be each made up of several 

 distinct grains, in the fissures between which the substance referred 

 to has obviously been deposited. The quartz of this sandstone occa- 

 sionally encloses a few needles of schorl or hair-like crystals of 

 rutile ; cavities containing bubbles are rare. 



A rock belonging to the Coniston-Grit series from Green-quarter 

 Pell, Westmoreland, consists of angular grains of quartz and felspar, 

 united by a siliceous cement traversed in all directions by numerous 

 greenish microliths. The average size of its constituent particles 

 does not exceed jjh'Q i ncn > although there are a few larger ones, 

 measuring about inch in diameter. The quartz contains few fluid- 

 cavities with bubbles ; but when these occur they are extremely 

 minute. This rock contains a little iron pyrites, and the cement is 

 sometimes stained by hydrated ferric oxide ; a few flakes of colour- 

 less and dark-brown mica are occasionally seen between the grains 

 of quartz and felspar. 



Devonian. — The majority of the siliceous grits of Cornwall are 

 usually regarded as being of Devonian age ; but it is probable that 

 some of them may be of older date. 



Two distinct beds of such rock, of a greenish-grey colour, which 

 are worked for road-metal, are quarried on the farms of Tregian and 

 Dairy, in the parish of St. Ewe, near St. Austell, and were noticed 

 in a previous paper under the name of slaty agglomerates *. 



In both these localities the grit contains angular fragments of a 

 soft clay-slate of a greenish-blue colour, and is exceedingly hard 

 and tough. The rock quarried at Tregian is composed of a mixture 

 * Quart. Journ. Geol. Soc. 1878, vol. xxxiv. p. 476. 



