HISTORY OF GRITS AND SANDSTONES. 



13 



Bootle bore-hole of the Liverpool water-works, was, with a large 

 number of others from that district, kindly furnished to me by Mr. 

 Charles E. De Ranee, of the Geological Survey, but was found to 

 be too friable to admit of the preparation from it of thin sections. 

 On examination by reflected light, however, it was found to be so 

 entirely made up of rounded grains, varying in diameter from -J^ 

 to -g-J-Q- of an inch, that I did not hesitate to suggest to Mr. De 

 Ranee the probability of its origin being due to blown sands united 

 by a ferruginous cement. 



These grains, of which the majority are quartz, are so rounded as 

 to represent miniature pebbles, while a few, consisting of partially 

 decomposed felspar, are often corroded into deep cavities on one or 

 more of their sides. 



The granules of quartz, as well as those of felspar, have been 

 covered by a thin coating of hydrated ferric oxide ; while on the 

 surfaces of the former a beautiful growth of crystals of trans- 

 parent quartz has frequently taken place. 



These crystals do not often exceed inch in length; but they are 

 sometimes very perfect, with sharply defined angles, and frequently 

 exhibit both plagihedral modifications and horizontal striation. A 

 few crystals of pyrites and of calcite have also been formed on the 

 surfaces of the rounded quartz-grains. 



Figs. 1, 2, 3, and 5, PI. II., drawn by Mr. P. Rutley from 

 specimens which I selected for that purpose, and magnified one 

 hundred diameters, represent crystals of quartz attached to rounded 

 grains of the same mineral forming the basis of this sandstone. 

 Pig. 4, magnified to the same extent as the others, is a grain of 

 quartz which exhibits a depression at its point of contact with 

 another similar body. Specimens presenting this appearance, which 

 are not very numerous, may sometimes be the result of one grain of 

 quartz having been forced or ground into the substance of another. 

 A careful examination, however, of such depressions leads to the 

 conclusion that in some cases when this pitting of a grain is ob- 

 served a deposit of silica may have taken place upon all parts of its 

 surface, excepting where it has been protected by contact with 

 adjoining grains. 



Angular cores of a siliceous material which have been deposited 

 in cavities at the point of junction of several grains are sometimes 

 detached when the rock is carefully disintegrated by friction with 

 a hard brush. When these adhere to one only of the adjacent 

 grains, having separated from the others, they obviously might give 

 rise to depressions of the kind referred to. 



If, after treating this sandstone by hydrochloric acid, the residue 

 be examined under the microscope, the presence of these siliceous 

 bodies becomes at once apparent. They are sometimes slightly 

 coloured by ferric oxide, and do not always exhibit colours when 

 seen in polarized light ; in other cases they afford evidence of im- 

 perfect crystalline structure, and are occasionally colourless and 

 transparent. 



Pig. 6, PI. II., is a grain of felspar which has become so corroded 



