271 



than the augite. It was brown in colour and occasionally possessed definite 

 form so far as the prismatic zone is concerned. It is now largely 

 represented by chloride alteration products with which epidote is 

 associated. The biotite is now green. Sections at right angles to the 

 principal cleavage exhibit a marked dichroism : a, pale yellowish brown, 

 /3 and 7 green. Sometimes the biotite is represented by feebly polarizing 

 chlorite ; as a rule the double refraction is stronger than it is in typical 

 chlorite. Ilmenite more or less changed to leucoxene and often exhibiting 

 the reticulated structure so common in the diabases is present in almost all 

 slides. Apatite occurs in long slender prisms. The idiomorphic felspar in 

 this rock has generally been rendered more or less turbid by the develop- 

 ment of vividly polarizing granular matter. Green secondary products 

 resulting from the decomposition of the bisilicates have sometimes migrated, 

 so to speak, into the felspars. The felspathic constituent of the micro- 

 pegmatite is often as fresh and unaltered as the quartz with which it is 

 intergrown. 



From the above description it appears that the original rock consisted 

 of plagioclase, augite, hornblende, iron-ores and a matrix of micro-pegmatite. 

 Alteration has given rise to uralite, chlorite, leucoxene and epidote. The 

 last-mentioned constituent occurs abundantly in all the slides and may 

 be often recognized macroscopically in nests and veins. Pyrite also occurs 

 pretty frequently. So far reference has been made to the coarse-grained 

 variety. Associated with this is a rock of finer grain. This is redder in 

 colour and appears to contain a smaller proportion of the ferro-magnesian 

 constituents. Messrs. HILL and BONNEY state that crystals of plagioclase 

 appear to be more abundant in this than in the other variety. 



The rocks from the northern portion of the district are shown by the 

 authors just mentioned to be closely related to those above described. 

 Somewhat similar rocks occur in the neighbourhood of Narborough, some 

 six or seven miles south of Groby. These may be regarded as inliers of the 

 Charnwood rocks. The rock of Enderby is interesting as illustrating a kind 

 of transition between the granitic and the trachytic textures. It consists 

 of more or less decomposed plagioclase crystals, corroded quartz-grains, 

 green decomposition products after the ferro-magnesian constituents, magne- 

 tite or ilmenite and a micro-crystalline mosaic of quartz and turbid felspar 

 which constitutes the ground-mass. The porphyritic felspar appears to be 

 wholly plagioclase. The porphyritic quartz has the aspect of the quartz 

 in the liparites and quartz-felsites. The rock differs from the dacites 

 merely in the fact that the ground-mass is micro-granitic. In com- 

 position it is evidently more acid than the normal rocks of Groby and 

 Markfield. 



Some analyses of Charnwood rocks have been made by Mr. BERRY. (I) 

 The silica percentage of the Markfield rock is given as 5678 ; that of the 

 Garendon rock as 5T54 ; and that of the rock from Huncote quarry, 

 Croft Hill, as 64-30. 



The augite-bearing rocks of Markfield, Groby and Bradgate possess 



~ (1) Q.J.G.S. Vol. XXXVIII. (1882), p. 197, 



