Vol. 54.] IX THE CAEBONIFEROUS LIMESTONE OP DERBYSHIRE. 171 



of detaelied blocks cover part of the slope on its north-western 

 side. The rock is of a dark colour ; its surface is often rough, and 

 contains a number of small holes. It shows no sign of stratification, 

 very hard, breaks with a glistening fracture, and has a crystalline 

 structure. In a hand-specimen it somewhat resembles a fine-grained 

 quartzite. 



Five specimens were collected, and thin slices were examined. 

 Wo. 8,^ sp. gr. 2'51, in ordinary light, is a granular rock, with 

 black or dark brown material, probably haematite, between the 

 grains. Between crossed nicols it appears as an aggregate of quartz- 

 grains, the majority of which are elongated in the direction of the 

 least axis of depolarization. They vary in size from less than 

 •02 X '01 mm. up to -04 x '12 mm. They have no crystalline 

 outline, but closely interlock and penetrate each other. The rock 

 is not formed of detrital grains cemented together by secondary 

 quartz, but appears to have originated by the crystallization of the 

 quartz in situ in such a manner that adjacent grains have prevented 

 their neighbours from assuming crystalline boundaries. Some of 

 the grains contain calcite. 



No. 420 is very similar, except that it contains a large quartz- 

 grain measuring 1*25 x '75 mm., and including smaller grains in it. 



No. 421, sp. gr. 2*60, has a similar structure. Several large 

 grains of quartz contain calcite. Small pieces of fluor fill a few 

 spaces between the quartz -grains. Vacant places in the section are 

 probably due to the fluor which filled them being broken out in 

 grinding. 



No. 422, sp. gr. 2*57, is similar to the preceding, but contains 

 a larger proportion of fluor. One cube of this mineral is seen 

 embedded in and penetrating three adjacent quartz-grains. The 

 quartz which penetrates, and is enclosed in, the irregular masses of 

 fluor in most cases possesses crystalline outline, giving either 

 hexagonal cross-sections, remaining extinct under crossed nicols, or 

 those nearly parallel to the axis extinguishing with their length, 

 and terminated at each end by the pyramidal faces. No. 423, 

 sp. gr. 2-53, has a similar structure. 



{b) ' The Old Chert-quany.^ 



About 600 feet north of the ' Top Lift ' is an old quarry marked 

 ' Chert-quarry ' on the 1 -inch Geological Survey map. This is probably 

 the ' Stubben' white chert or limestone-quarry mentioned by Parey.^ 

 An almost vertical face of rock, about 13 feet high, is exposed, in 

 which no chert is seen. The face consists mainly of a quartz-rock 

 of the ' Top Lift ' type, passing in places into a quartzose lime- 

 stone, which is often traversed by thin veins of quartz and some- 

 times of calcite. The quartz-rock was not examined microscopically, 

 because it was obviously similar to the ' Top Lift ' rock. 



^ These numbers refer to the thin slices examined under the microscope. 

 - ' General View of the Agriculture & Minerals of Derbyshire,' vol. i (1811) 

 p. 273. 



