PEECARBOELFEROUS ROCKS OF CHARNWOOD FOREST. 221 



Sheet-Hedges Dyke. — This dyke is a contact, dull green rock, of 

 a slightly mottled aspect, and rather irregular fracture, weathering- 

 brown. It appeared to be about two feet thick ; the junction was 

 not very well exposed when Ave visited it. Under the microscope 

 the ground-mass is a clear glass, thickly crowded with minute pale 

 brown and green microliths, and traversed by a sort of irregular 

 network of viridite, much of it the dichroic bluish variety, so that 

 it is divided up into ill-defined rounded patches. Grains of magne- 

 tite (?), larger patches of viridite, and a good many long felspar 

 crystals are scattered about. With crossed Nicols these rounded 

 patches become more conspicuous, recalling a structure not infre- 

 quent in some lavas ; the whole rock, however, is now a mass of 

 pseudomorphs, all the felspathic element being apparently converted 

 into secondary microliths ; a very little quartz, probably Of secondary 

 formation, and a little dolomite (?) are also present. The rock may 

 have been an andesite. 



Bradgate-Park Dyke. — This dyke is well seen in the bed of the 

 stream about one hundred yards below the last fish-pond ; it is in 

 general rather less than two feet wide, with distinct, irregular, and 

 closely welded junctions, sharply and finely jointed, tolerably sub- 

 conchoidal fracture, dull grey green-colour with lighter specks, and 

 occasional distinct crystals of pinkish felspar. It is a rather heavy 

 rock, weathering brown. 



Under the microscope it appears to have a glassy base, thickly 

 crowded with microliths of felspar or its pseudomorph ; in this lie 

 many larger crystals of felspar, apparently of both kinds (partly 

 pseudomorphosed), the usual dusky grains, patches of viridite, pro- 

 bably replacing hornblende, and some calcite with a little quartz. 

 The rock is probably an altered hornblende andesite. 



Mount-Sorrel Dykes. — West of the principal quarry, a little below 

 the summit of the hill, a dyke of compact pinkish-red felstone can be 

 traced for some eighty yards. It strikes about ISLE., dips at an angle 

 of about 60° towards the eastern side, and is generally about six 

 to eight inches thick ; in two or three places it splits and includes 

 small slabs of granite, in others thrusts in a small tongue. At the 

 junction it sometimes flakes away from the granite, sometimes adheres 

 firmly. 



The base of the felstone dyke appears under the microscope to be a 

 true glass, but very thickly crowded with granular felspar microliths, 

 together with grains of iron oxide and a few needles of hornblende, all 

 very minute. Somewhat larger crystals of felspar, orthoclase, with 

 some plagioclase, are scattered about, their longer axes roughly parallel 

 to the junction surface, and a little quartz ; there are a few imperfect 

 spherulites. The junction with the granite is clear and sharp, and 

 one or two little fragments of it are included. This rock is not more 

 ancient in aspect than some of the specimens of Hungarian trachytes, 

 with which it might almost be classed*. 



On the floor of the pit were picked up fragments of a thin vein of 



* Mr. Houghton has determined for us the percentage of silica ; it is 77*7, 

 which proves the rock to be a true rhyolite. 



