16 



J. A. PHILLIPS ON THE CONSTITUTION AND 



which crystals of schorl are also found owe their origin to a different 

 source from that whence the supply of quartz which does not contain 

 such cavities was derived. 



Jurassic. — A fine-grained Upper Lias sand, of a greenish colour, 

 from Seizincote, Stow - on - the -Wold, Gloucestershire, effervesces 

 when treated with hydrochloric acid, leaving grains of transparent 

 quartz, which are generally angular. In some instances, however, 

 their more acute angles appear to be slightly rounded, although 

 the mean diameter of the fragments is only -^hl inch. Besides 

 quartz containing occasional fluid-cavities, usually without hubbies, 

 there are present a few pieces of somewhat doubtful felspar, together 

 with numerous fragmentary crystals of schorl and garnet. 



The only coherent arenaceous rock of this age which I have had 

 an opportunity of examining is that quarried at Egton, near Whitby, 

 under the name of " Moor Grit," which is locally much employed 

 for road-metal. It is white and fine-grained, being often so 

 compact as to be entitled rather to the name of quartzite than to 

 that of grit. Its geological horizon is above the Grey Limestone in 

 the estuarine series of the Lower Oolite in North-east Yorkshire. 



Under the microscope this rock is seen to be almost entirely 

 composed of transparent, colourless quartz, of which the largest 

 pieces are about y 1 ^ inch in diameter, and of which the angles are 

 usually more or. less removed. Around and between these grains 

 a deposit of transparent crystalline quartz has taken place, thus 

 forming a cementing medium. A few small garnets are present, 

 but no fluid- cavities with bubbles were observed, although some of 

 the quartz encloses minute crystals of a yellowish mineral which 

 I have been unable to identify ; these are exceedingly minute, 

 often not exceeding yulnny ^ ncn * n length. Many of the smaller 

 grains in this rock exhibit, when examined in polarized light, that 

 complex structure so frequently observed in the quartz of clay-slates 

 and other somewhat similar rocks. 



A sand resulting from a disintegrated Portland Stone at Fonthill 

 Giffard, Wiltshire, is largely composed of ovoid grains of calcite. 

 After being attacked by hydrochloric acid, a rounded quartzose sand, 

 amounting to about one quarter of the total bulk of the mixture, 

 remains behind. This sand, of which the grains vary from y 1 ^ to 

 inch in diameter, contains but few fluid-cavities, and these, as a rule, 

 are without bubbles. The fragments of quartz are associated with, 

 and not enclosed by, the ovoid grains of calcic carbonate. 



Cretaceous. — The Tilgate Sandstone, Ashdown Sands, from Newick 

 Park, Sussex, is composed of slightly rounded grains of colourless 

 transparent quartz, united by a cement consisting partly of calcic 

 carbonate and partly of flint. The quartz is almost entirely free 

 from fluid-cavities, but encloses a few hair-like crystals of a mineral 

 which is probably rutile. If felspar be present it has become too 

 extensively altered to admit of identification. 



Sections have been examined of the chert known as Sevenoaks 

 Stone, as well as of several others of Lower Greensand age. They 

 all contain numerous fossils, particularly sponge-spicules, and in 



