OOLITE OF THE COriESWOLD HILLS. 565 



{ind the freed silica as having become soluble in water, and as having 

 been subsequently redeposited, either replacing the carbonate of lime 

 forming the tests or skeletal structures of organisms, or infilling the 

 spaces rendered vacant by the removal of organic matter as the 

 result of decomposition. The evidence in support of this is the fact 

 of the occurrence of silicate of alumina with the amorphous and 

 cry])tocrystalline silica. 



There is another mineral seen in the residues to which as yet 

 I have made no allusion. It occurs in the form of angular chips ; 

 they are isotropic, colourless, or slightly tinged with pink, and 

 especially remarkable for their high order of refraction. Mr. W. M. 

 Hutchings, in his article " Notes on the probable Origin of some 

 Slates from the Cliffs near the Village of Seaton " *, says : — " The 

 minerals which fell out were zircons, numerous and of rather large 

 size, garnets in angular colourless fragments," &g. The part of this 

 statement to which I desire to draw special attention is that which 

 refers to the garnets. The description corresponds with the chips 

 of that mineral in the residues, except that in some cases they are 

 tinged with pink. 



(c) Origin of the Residues. 



The detrital material now in the rocks cannot be taken to repre- 

 sent the quantity at first deposited. Chemical changes have taken 

 place by which some original materials have been decomposed and 

 others constructed. It is, therefore, only the most indestructible con- 

 stituents of the original minerals which remain, and among these 

 are grains of quartz. This is shown by the rounding action of the 

 water, which is as clearly defined on the larger grains as at the time 

 when it took place (see PI. XX. fig. 5). 



The quantity of felspar present in some residues seems to indicate 

 that there must have been a considerable amount of felspathic 

 minerals in the sediment. It is probably owing to the decomposition 

 of these that the silicate of alumina in the strata has been derived 

 and the argillaceous character of some beds is due. 



The origin of the minute crystals of rutile noticed in the Oolite 

 Marl and Argillaceous Bed. at the top of the Upper Freestone at 

 Chedworth is a question of considerable interest. Rutile is known 

 to occur as a secondary product in some instances ; but in this case 

 I think the evidence is in favour of its having been derived as 

 detrital material from other rocks, possibly in part as inclusions in 

 other minerals. In support of this hypothesis I may point out that 

 the quartz-grains in the Oolite Marl and Clay Bed only average 0*09 

 and 0-08 mm. in longest diameter respectively. It would seem j)ro- 

 bable, therefore, that the detrital material originally deposited was 

 very fine, and it would be with this detrital material. that the rutile 

 would be carried. Other crystals might be expected to separate 

 in situ by the decomposition of the minerals containing them as 

 inclusions. 



I regard the detrital residue as a whole as having been derived 

 * Geol. Mag. (1890) p. 20G. 



2r2 



