26 



J. A. PHILLIPS ON THE CONSTITUTION AND 



inches in length, is now in the collection of the Museum of Practical 

 Geology. Sandstones of this description are not unfrequently -with- 

 out any kind of cementing matrix, being merely felted together by 

 a matted intergrowth of their constituent crystals. 



Professor Daubree entertains the opinion that crystalline sand- 

 stones frequently owe their origin to chemical agencies resulting 

 from an outpouring of igneous rock ; but, although this may some- 

 times have been the case, many of the most completely crystalline 

 British sandstones are situated at distances of many miles from any 

 known rock belonging to this class. The same author maintains 

 that the presence of anhydrous ferric oxide in sandstones affords 

 evidence of their having been subjected to a high temperature*. It 

 must, however, be remembered that the carnallite of Stassfurt, which 

 has evidently never been highly heated, contains crystals of specular 

 iron-ore. 



Numerous fine-grained sandstones, particularly among those of 

 Triassic Age, are composed of quartz grains so completely rounded 

 as, under the microscope, to resemble well-worn pebbles. These 

 " millet-seed" sandstones are often coloured either red or brown 

 by variously hydrated oxides of iron ; and in some cases minute, 

 perfectly formed, and beautifully transparent crystals of quartz have 

 been developed upon their surfaces. 



On attacking the sand of such sandstones with hydrochloric acid, 

 the oxide of iron is easily removed, but the crystals of quartz still 

 remain firmly attached to the surface of the grains upon which they 

 have grown. It would also appear that crystals have been formed 

 upon those parts only of the grains which, having been free from a 

 coating of oxide of iron and from every other extraneous material, 

 have admitted of direct chemical contact between the silica of the 

 rounded quartz and that of the subsequently formed crystals f. 



In addition to silica in the form of perfect crystals of quartz, that 

 substance has often been deposited in such a way as to fill cavities 

 existing between the original grains of sand. 



As this silica has frequently been thrown down upon a thin deposit 

 of ferric hydrate, it is generally detached by prolonged digestion 

 in hydrochloric acid, by which the intervening ferruginous coating of 

 the grains is ultimately removed. When such a deposit of silica 

 adheres to only one of the adjoining grains it may give rise to a de- 

 pression upon its surface of the kind represented in fig. 4, PI. II. 



On examining a considerable number of modern sands, none of 

 them, excepting such as had long been subjected to the wearing 

 effects of wind action, were found to resemble those of the millet - 

 seed sandstones in having all their grains reduced to a pebble-like 

 form. Among these the grains of blown desert-sands most com- 

 pletely resemble those of millet-seed sandstones. 



* Geologie Experimental?, pp. 226-230. 



t With regard to the Torridon Sandstones of the Central Highlands it has 

 been observed by Professor Bonney that wherever "dirt" has been deposited 

 npon the siliceous grains their agglutination has been prevented (Quart. Journ. 

 Geol. Soc. vol. xxxvi. p. 106). 



