Cherts and Dolomites. 35 
“No, 325. R. 93 Ripee.—(Overlying cherty beds.) —Is not 
very different from 303, except in the alteration of the 
rounded grains. In many cases, these consist of a very 
dark reddish-brown micaceous substance, mingled with a 
green mineral (probably of the serpentine group) and 
reddish-brown iron hydroxide. In some of the lighter 
colored grains, the remains of a colorless augite can readily 
be detected. 
“In other cases, the entire substance of the original grain 
has given place to silica in the form of a fine mosaic of 
quartz. In these, the outline of the original grain has been 
rendered permanent by a line of little plates of brown mica. 
“ As in 303, the interstitial substance is quartz. Around 
the edges of the separate grains, crystals of quartz extend 
out on all sides, like the lining of a vein. Where the space 
between the fragmental grains was small, the two rows of 
quartz crystals mutually interfere and completely fill the 
spaces ; where the intervening space was large, that portion 
in its centre between the rows of crystals is filled by a 
mosaic of the same mineral. Cracks which extend through 
the rock contain iron oxides or hydroxides.” 
A number of other sections examined by the writer 
presented in a general way the same characteristics as those 
above described. All without exception shewed the feath- 
ery periphry of quartz crystals, with the mosaic of larger 
individuals in the larger spaces. The highly ferruginous 
varieties, shewing magnetic polarity, were seen to be very 
largely composed of magnetite, generally disposed around 
the rim of the particles, the central parts being mostly 
composed of a finely crystallized quartz mass. Many 
particles, however, seem to be almost wholly made up of 
the iron minerals. By reflected light the ferruginous areas 
gave, in places, a dark red colour, shewing the alteration of 
the magnetic into ferric oxide. Although not a frequent 
phenomenon, the magnetite was occasionally seen to be 
deposited in the interstitial quartz areas. In these cases 
the mineral would be found to fill the centre of the space, 
its outlines conforming to those of the surrounding grains, 
