PRECARBONIEEROtTS ROCKS OF CHARNWOOD FOREST. 219* 



ground-mass becoming more finely crystalline. Sometimes it is 

 much decomposed near the surface, and the hill is found to be tra- 

 versed by two (or more) nearly vertical bands of rotten rock (one 

 15 to 2o feet wide) defined by joints running roughly north and south 

 called "mush by the quarrymen. There are generally three or 

 four systems of joints cutting the rock into rather irregular masses, 

 the best marked being the one running as above. In the lower 

 part of the pit these joints are so well defined as to give the rock quite 

 a platy aspect, and are cut by another set, further apart, sloping at 

 about 60 *. These, as we were shown, dipped rather to the west 

 higher up the hill, and on the other side of the crest were reported 

 to dip steeply m the opposite direction. The rock is often slicken- 

 sided; and the joints coated with mineral substances, resembling 

 sometimes haematite, sometimes chlorite. Pyrite is seen in the rock 

 and veins, and nests of quartz and epidote, also sulphate of baryta 

 bitumen, and a little carbonate of copper. Mr. Harrison states that 

 molybdenite has been found. 



Microscopic Structure of Mount- Sorrel Granite.— The red and grey 

 varieties have both been examined. The former consists of well- 

 crystallized quartz, felspar, biotite, hornblende, and magnetite, with 

 a little apatite. The quartz and felspar are occasionally intercrys- 

 tallized so as to offer a slight approach to the graphic structure 

 already described. The quartz contains minute cavities and micro- 

 liths but is otherwise clear, and shows brilliant colours with crossing 

 £hcols Some of the felspar is a good deal decomposed, and is replaced 

 by zeolite mixed with brown dust. Most of it appears to be ortho- 

 clase, but, as m the syenite, there is a fair quantity of plagioclase, pro- 

 bably oligoclase, which seems to have escaped better. Some crystals 

 exhibit zonal banding. The hornblende is not very characteristic is 

 a good deal decomposed, and partly replaced by epidote. The biotite 

 has predominated, part of it being very well preserved. Some crystals 

 are more or less replaced by a clear greenish mineral, strongly 

 oichroic (in a few instances showing aggregate polarization), seem- 

 ingly of the same crystalline system; its action, however as 

 an analyzer is comparatively slight, the pseudomorph only very 

 slightly darkening its colour, while a biotite crystal so used when 

 lying m the proper position is almost black. The grey variety is 

 under the microscope, almost undistinguishable from the last the 

 difference m colour being apparently due to a different tinge in the 

 earthy part of the decomposed felspar. One of the dark, apparently 

 included, fragments has been examined. It consists of crystalline 

 quartz m tolerably large grains, in which are irregularly scattered 

 longish prismatic crystals of decomposed felspar and of fibrous 

 hornblende, numerous grains of magnetite, and very many belonites ; 

 these last are colourless, and generally less than -01 inch long. So 



th*Jw !° rem u n . t0ld US , that ^ e r0ck had a kind of "cleavage" Parallel to 

 these joints, so that a workman who knew the pit could make in an hour or two 

 as many paving-sets as a stranger could in a day. Also that this "cleavage" 

 on approaching the joint-planes curved a little towards them, clearly indicTtL 

 some approach to a fissile structure. "" ? "miutung 



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