LOWER PRIMARY SANDSTONES. 95 



crystals of rutile, and the cement contains calcite, magnatite, iron 

 pyrites, and a few imperfect garnets. Near Harlech, the sands are 

 liner and purpler, but only differ in the cement containing crystals of 

 epidote. When analysed, silica forms 80 per cent, of this rock. The 

 well-known sandstone of the Stiper stones west of the Longmynd, near 

 Shrewsbury, has grains with an average diameter of one-fiftieth of an 

 inch ; some, rounded almost like pebbles, are converted into perfect 

 crystals by a deposit of transparent quartz upon the sand nucleus, and 

 the grains are generally so closely cemented by crystalline silica as to 

 form a quartzite. The few grains which contain fluid cavities usually 

 have the cavities full. Felspar is not abundant. 



The grey grit of Aberystwith consists of nearly equal proportions 

 of quartz grains and felspathic grains cemented by silica. Some of 

 the grains are the -^th of an inch in diameter, generally rounded, 

 though a few are sharp ; the quartz contains very minute fluid cavi- 

 ties which are generally full, though some enclose moving bubbles, 

 and some crystals contain needles of tourmaline. The felspar is partly 

 triclinic, and fragments of a volcanic rock like basalt occur ; in the 

 silicious cement are small flakes of mica and a few crystals of iron 

 pyrites. This rock has been derived in part from the disintegration 

 of quartz felsite. 



The May-Hill sandstone is chiefly composed of angular grains 

 o-J^th of an inch in diameter, united by a turbid silicious cement ; 

 there are a few larger grains, these like the smaller ones contain hair- 

 like crystals of rutile, but fluid cavities with bubbles are rare. In 

 the Lickey Hills, in Worcestershire, the rocks of this age have 

 the grains greatly rounded, the diameter of -g^th of an inch; fluid 

 cavities with bubbles are numerous in some of the grains, absent in 

 others ; rutile and tourmaline both occur in the quartz in minute 

 crystals. 



The Denbigh grit consists chiefly of a fine-grained cement con- 

 taining both minute and larger fragments of quartz with felspar and 

 brown mica ; and the quartz sometimes encloses needles of tourmaline, 

 sometimes crystals of rutile. There are few cavities with bubbles. 



In the Devonian rocks the grits near St. Austell are a mixture of 

 angular pieces of quartz and felspar ; the quartz includes crystals of 

 tourmaline and few fluid cavities. The felspar is partly orthoclase, 

 partly triclinic, and the rock contains a little silvery mica and a few 

 crystals of pyrites. Specimens from Ladock enclose many angular 

 fragments of a greenish slate; these are included among small rounded 

 grains of quartz, felspar, and other substances, among which are pieces 

 of volcanic rocks closely resembling the Cornish greenstones and dun- 

 stones ; but a second specimen from the same locality is chiefly made 

 of angular fragments with more abundant fluid cavities in the quartz, 

 some grains of which contain epidote and flakes of white mica. The 

 felspar is chiefly triclinic, and there are a few minute garnets and 

 some fragments of organic rocks. These data sufficiently indicate the 

 varied nature of the materials from which the Devonian grits of 

 Cornwall were derived. 



