Vol. 56.] BTJNTEE PEBBLE-BEDS OF THE MIDLANDS. 293 



Dark Pebbles. — In my last paper I referred to certain pebbles, 

 compact to minutely granular, varying from dark green to black, 

 more often under than over 2 inches in diameter. To these of 

 late years I have paid more attention. Microscopic examination 

 proves them to include more than one kind of rock, some of which 

 I proceed to describe. 



A fairly rounded, minutely granular, black pebble (Style Cop) is a 

 fine-grained grit composed of angular quartz-fragments, among 

 which are one or two minute zircons, and interstitial matter so 

 blackened as to conceal its exact nature, with a few (secondary) 

 crystals of iron-oxide. Another (North Pit, Style Cop) about 2 inches 

 in diameter, dark purplish-brown and minutely granular, shows 

 distinct grains of quartz, generally angular, outlined by black 

 granules, which also permeate the interstitial quartz. A third 

 pebble from the same hill resembles a fine-grained quartzite, with 

 a few very minute flakes of colourless mica, brecciated and then 

 cemented by minutely crystalline quartz, full of opacite. 



A fourth specimen (road on Racecourse) consists mainly of 

 quartz (large subangular grains in a microcrystalline matrix) and 

 minute flakes of a rather dark micaceous mineral ; the whole sug- 

 gesting a rock so much crushed as to assume a ' mylonitic ' structure. 



Two small dark pebbles (one from the Satnall Hills, the other 

 from the Chase near Rugeley) resemble chert, traversed with minute 

 quartz-veins. The structure under the microscope is that of a chert, 

 and the former one is somewhat distinctly banded. 



In both we see a number of small bodies circular or oval in shape, 

 the inner part being slightly different from the outer. Suspecting 

 these to be very ill-preserved radiolaria, and ^wishing to make 

 sure, I trespassed (not for the first time) on the kindness of 

 Dr. G. J. Hinde, F.R.S., and met with the usual generous response, 

 which is to the following effect : — ' The rounded bodies in the slice 

 from the Satnall Hills are, in my opinion, casts of radiolaria, and some 

 of the smaller circles may be cross-sections of sponge-spicules. I do 

 not see any structure preserved, but this happens, unfortunately, in 

 the large majority of radiolarian cherts. The only character which 

 differentiates the slice from the usual radiolarian chert is that 

 the circular bodies are generally smaller than the casts in the Palaeo- 

 zoic radiolarian cherts that I have hitherto examined. Many of 

 the casts, moreover, seem to have been squeezed out of shape. 

 The other slide appears also to be a true radiolarian chert.' 

 (See PI. XX, fig. 5.) 



Other dark pebbles contain a fair quantity of tourmaline. 

 Occasionally it is very minute, rather filmy in aspect, not strongly 

 pleochroic, and of a pale brown colour. But a little acicular indigo- 

 tourmaline commonly can be detected, and is usually well-developed 

 in any vein. Possibly the mineral of the matrix may be sometimes 

 a brown mica, and I think it very probable that tourmaline often 

 has replaced that mineral. One pebble (Style Cop) appears to have 

 been a rather fine-grained felspathic quartz-grit, in which the 

 aluminous constituent has been converted into a granular brown 



