HISTOST OF GRITS AXD SANDSTONES. 



17 



some cases consist, to a large extent, of amorphous silica ; in others 

 the rock becomes crystalline and distinctly chalcedonic. The flinty 

 varieties often contain crystals of calcite, which cluster around an 

 included fossil as a nucleus. All the specimens examined contain 

 glauconite, and occasionally grains of ordinary quartz, some of which 

 are much rounded, while others are angular. 



The " Carstone of Hunstanton, near King's Lynn, Norfolk, and 

 of various other localities, is a friable ferruginous sandstone belonging 

 to the upper portion of the Lower Greensand formation, and occurring 

 in beds of which the relations have not, as yet, been accurately 

 determined. A series of specimens from this locality was kindly 

 furnished me by Mr. S. B. J. Skertchly, of the Geological Survey, 

 who is at present occupied in working out the geology of the district. 

 "With the exception of certain variations in colour, these beds so 

 closely resemble one another in their general characteristics that a 

 description of one of them will suffice for the present purpose. 



A specimen of Carstone from immediately below the Red Chalk 

 exposed in the cliff at Hunstanton was found to be mainly composed 

 of a mixture of somewhat rounded grains of quartz, with small 

 pebble-like granules of dark-brown iron-ore. The individual grains 

 of these minerals vary in diameter from yL- inch to the most minute 

 sand, although small pebbles of larger size than the highest limit 

 quoted are not of unfrequent occurrence. The quartz contains 

 schorl and rutile, together with a few fluid- cavities, of which the 

 majority are without bubbles. In addition to ordinary quartz 

 grains, this rock, when carefully disintegrated, exhibits numerous 

 examples of the angular bodies resulting from the breaking-up of a 

 siliceous deposit formed between the grains of the original sand, 

 which have been noticed (p. 13) in connexion with millet-seed 

 sandstones of Bunter age. 



By the prolonged action of hydrochloric acid the quartz of this 

 rock is rendered colourless, while the globules of ferric hydrate are 

 dissolved, with the exception of a siliceous skeleton which preserves 

 the exact form of the original grains. These bodies do not usually 

 exhibit colours when mounted in balsam and examined by polarized 

 light ; but in a few cases the presence of a dark cross indicates a 

 pisolitic structure in the siliceous residue*. In addition to the 

 foregoing, this rock contains a few minute scales of mica and a very 

 small quantity of felspar. The majority of the grains of quartz 

 have their angles distinctly abraded : in some instances they 

 have been completely removed, and a pebble-like form has been the 

 result. 



A specimen of Carstone obtained from a bed directly beneath the 

 Red Chalk, afforded on analysis the following results : — 



* Both Dr. Percy and Professor Judd hare described siliceous skeletons which 

 occur in the pisolitic grains of Northamptonshire iron-ore : — Metallurgy, Iron 

 and Steel, pp. 225, 226 ; Memoirs of the Geological Survey, Geology of Rutland 

 &c. p. 119. 



Q.J.G.S. No. 145. 



c 



