564 MR. E. WETHERED ON THE INFERIOR 



an analysis was made with the following? result, after drying at 



212° E.:— 



Loss on ignition 9'66 



Silica 67-90 



Alumina with a trace of iron 1800 



Magnesia 006 



Other constituents, by loss 4"38 



10000 



The specimen was taken from the Barrow cutting on the Midland 

 and South-Wcstern Junction llailway near Chedworth. 



As in the case of the Oolite Marl, there is associated with the 

 Argillaceous Bed a quantity of very small mineral fragments, among 

 which were the extremely minute crystals of rutile, known as 

 the " clay-slate needles." 



The Clyj)eus Plotii Beds. — A typical specimen from these beds 

 near Chedworth gave 5 per cent, of residue consisting of quartz and 

 felspar, some of the latter much decomposed. One crystal of zircon 

 was noticed. 



(b) General Summary of the Residues, 



Taking the residues as a whole, they may be said to contain 

 chiefly detrital quartz, and next in quantity felspars, zircons, tour- 

 maline, and occasionally rutile. In the Horsepools district, the 

 quartz-grains form almost the entire residue. The zircons may be 

 said to be most plentiful in the Transition Beds ; they occur in the 

 form of short prisms and show no sign of water-action. In some 

 beds the felspar is plentiful, and remarkable for the good state of 

 preservation of the fragments. Both monoclinic and triclinic fel- 

 spars are present, especially microcline. The micas occur in small 

 jagged flakes, chiefl)^ as muscovite. The quartz-grains are rounded, 

 except in the case of very minute ones ; and some contain in- 

 clusions. 



Several residues from the limestones contain silicate of alumina 

 in the form of scales, enclosing minute flakes of mica and quartz. 

 These scales were, however, formed by the coagulation of the silicate 

 during the process of drying the residue, a circumstance to which I 

 have directed special attention when describing the Oolite Marl 

 (see p. 562). 



In the Argillaceous Beds, silicate of alumina is plentiful in the 

 form of extremely minute granules. It would moreover appear 

 from Bed No. 5 in the Clceve-Hill section (PI. XX. fig. 6) that 

 silicate of alumina does undergo decomposition, the silica assuming 

 an amorphous form which subsequently becomes crystalline. It 

 may be, however, that the freed silica at first became soluble in 

 water, and that it was redeposited in the amorphous form, which 

 afterwards became crystalline. 



It is in this way that I account for the siliceous casts of organic 

 structures which occur in the typical Pea Grit, and in some beds of 

 the Upper Preestone. I am disposed to regard these cryptocrys- 

 talline siliceous casts as derived from the decomposition of silicates, 



