8 



J. A. PHILLIPS ON THE CONSTITUTION AND 



like among them form the nucleus of a crystalline growth of colour- 

 less transparent quartz, which converts them into more or less perfect 

 crystals of that mineral. Felspar is not abundant, and is often 

 considerably altered ; the cementing material occasionally contains 

 minute crystals of a mineral which is probably epidote. 



A grit of a grey colour, speckled with minute white points, from 

 the neighbourhood of Aberystwith, is seen under the microscope to 

 be composed of an almost equal amount of quartzose and felspathic 

 grains united by a siliceous cement, everywhere permeated by a 

 moss-like chloritic mineral, to which reference has already been 

 made. In this rock the constituent fragments are sometimes as much 

 as inch in length, and their angles are usually to a certain extent 

 rounded, although they occasionally exhibit very sharp and irregular 

 outlines. The quartz, which exceptionally contains a few needles 

 of schorl, is colourless and transparent, containing exceedingly 

 minute fluid-cavities, of which the majority are full, although others 

 enclose constantly moving bubbles. 



The felspar has been subjected to considerable alteration, and is 

 not unfrequently obscured by flocculent microliths resulting from 

 chemical re-combinations ; a portion of it, however, evidently belongs 

 to a triclinic species. In addition to the above, there are inclusions 

 which are undoubtedly fragments of a volcanic rock of basaltic 

 character. 



The siliceous cement contains a few crystals of iron pyrites, as 

 well as small flakes of brown and colourless mica, of which the 

 edges are much rounded. 



It is evident that some portion, at least, of the quartz constituting 

 this grit has been derived from the disintegration of quartz- felsite 

 (quartz-porphyry), since its general characteristics are not only 

 similar, but it, moreover, includes the blebby masses of an amorphous 

 ground-mass so characteristic of the quartz of such rocks. 



A fine-grained foliated rock belonging to the same series, from 

 Llangrannog, sometimes locally called a grit, of which the grains 

 vary from -g-J-^ to xuV"© - ^ ncn * n diameter, has a composition gene- 

 rally similar to that of the foregoing. In addition, however, it con- 

 tains numerous water-worn flakes of mica, which occur chiefly in 

 distinct bands, and which are arranged with their cleavage-surfaces 

 parallel to the plane of foliation of the rock. The fragments of 

 quartz, which are of a nearly uniform size, are all sharply angular, 

 and elongated or flattened grains are rare. 



An examination of sections prepared from May-Hill Sandstone, 

 containing numerous casts of Pentamerus oblongus, shows that this 

 rock is mainly composed of angular grains about -^io i ncn ^ n dia- 

 meter, united by a turbid siliceous cement, suggesting the idea of 

 its having been deposited from waters holding clay in suspension. 

 In addition to the smaller grains, of nearly equal dimensions, there 

 are a few fragments of, at least, four times the size above stated. 

 These, like the smaller ones, sometimes contain a few hair-like 

 crystals of rutile ; but fluid- cavities containing bubbles are exceed- 

 ingly rare. 



