171) 



reality diallage, and that the rock must therefore be classed with the 

 gabbros. (l) An examination of a considerable number of microscopic sections 

 shows that a rhombic pyroxene occurs sparingly ; but not in sufficient 

 abundance to remove the rock from the normal gabbros. 



The rock is found on the south side of Carrock Fell. It is well exposed 

 in the crags to the north of the small village of Mosedale. A striking feature 

 of the mass is its variability in composition. Some specimens consist for the 

 most part of ferro-magnesian constituents, others are mainly formed of 

 felspar in association with quartz. The basic rock, as already stated, passes 

 by insensible gradations into the acid augite-bearing graiiophyre (spherulitic 

 felsite of WARD), which forms the actual summit of Carrock Fell. The mass 

 not only shows variations in composition but also variations in texture. 

 Some portions are coarsely crystalline and granitic, showing conspicuous 

 crystals of diallage on the weathered surfaces ; other portions are fine grained 

 and ophitic in texture. Mr. WARD was much impressed with the existence 

 of a kind of bedding in the mass a little south of the summit of Carrock Fell. 

 Thus he says : " Alternating with very coarsely crystalline layers are others 

 of a very fine grain ; the aspect of the rock from a little distance quite 

 resembling the bedding among the volcanic series. Moreover, the dip of this 

 bedding is tolerably constant along a line of strike answering to the strike 

 of the volcanic series on the north, the dip being in this case north-wards, 

 at angles of from 50 to 70. Again, at the crags just north of Mosedale, 

 there are most undoubted cases of beds of contemporaneous trap, alternating 

 for short distances with hypersthenite, and passing into it along the east and 

 west line of strike." 



The mass is essentially composed of diallage, plagioclase and magnetite 

 or ilmenite. Quart/ is frequently present in the comparatively unaltered 

 rocks, and there seems no reason to doubt that it was largely an original 

 constituent. A rhombic pyroxene with distinctly marked pleochroism may 

 be detected in some specimens. Biotite is common in some slides. The 

 alteration products are uralite, actiiiolite, chlorite and other green minerals, 

 together with ill-defined substances arising in consequence of the alteration of 

 felspar. 



The monoclinic pyroxene in a few cases shows the two prismatic 

 cleavages only, and is then in the condition of augite. As a general rule, 

 however, it exhibits a lamination parallel to the orthopinacoid (100), and 

 contains the minute rod-like and tabular inclusions characteristic of diallare. 



o 



The transition from the augitic to the diallagic condition may be observed 

 in one and the same individual, as also the change from pyroxene to 

 hornblende. The diallage is usually a pale brown in thin section ; but in a 

 few cases it exhibits a pinkish tint. Cross sections, showing an optic axis in 

 convergent light, frequently exhibit a third cleavage parallel to the optic 

 axial plane (010), as well as the two prismatic cleavages and the diallagic 

 striation. In one case a clino-piacoidal section of a twinned crystal showed 



(1) G.M. Decade II., Vol. IX., 1882 .p. 210. 



