THE SHAr GRANITE AND ASSOCIATED HOCKS. 27^^ 



probably due to enclosed crystals of plagioclase ; but, making liberal 

 allowance for this, the potash- and soda-felspar molecules must be 

 combined in the mineral in some such ratio as 4:1. The t'els])ar 

 resembles tlic dominant one in the granites of Leinster, investigated 

 by Dr. S. Haughton, and the analysis of one of these is here quoted 

 for comparison (II.)- 



Dr. Cohen has also analysed the groundmass of the granite : that 

 is, the rock excluding the porphyritic felspars. From his figures 

 (III.), and remembering that part of the potash must be contained 

 in the mica, we see that among the smaller felspars of the rock 

 plagioclase is the dominant variety. Comparing the figures in 

 columns I. and III. with the bulk-analysis of the rock given above 

 (p. 270), it is seen that the porphyritic crystals constitute about 

 one-tenth o£ the whole mass of the rock (see column II. on p. 276). 



The plagioclase felspar occurs in idiomorphic crystals, often en- 

 closing zircon, dark mica, &c., but moulded by the quartz and later 

 orthoclase ; these facts sufficiently fix the time of formation of the 

 mineral. Albite-twinning is always seen, the lamellrc being rather 

 narrow. Carlsbad-twinning sometimes occurs in addition [395 a], 

 and more rarely a lamellation answering to the perjcline law r39o, 

 876]. The optical properties point to oligoclase. The crystals are 

 frequently turbid, being filled w^ith a fine dust doubtless due to de- 

 composition, and calcite is also to be detected, besides minute fan- 

 like groups of fibres, probably of some soda-zeolite. 



The quartz and later orthoclase call for no special remarks. The 

 last-named mineral, being commonly the latest product of consolida- 

 tion, is for the most part without crystal boundaries, and moulds 

 the irregularly shaped or rounded grains of quartz (see PI. XI. 

 fig. 1). Micropegmatitic intergrowth of the two is not found in the 

 normal type of granite. 



The structure of the Shap Fell granite seems to warrant some 

 inferences as to the conditions under which it was injected into its 

 present position. The intrusion must have occurred soon after the close 

 of the Silurian period. Taking the thickness of the Silurian strata 

 as 14,000 feet, we obtain an approximation to the depth at which 

 consolidation took place. By the consolidation of an igneous rock 

 we must understand the consolidation of such of its constituents as 

 crystallized in situ, and in particular of the one last formed, which 

 in this case is the later generation of orthoclase. Earlier-formed 

 minerals may have separated out from the magma at greater depths 

 and been carried up to their present position. From a study of the 

 fluid-cavities enclosed in the quartz of this rock, Mr. Clifton AVard 

 deduced that it was formed under a pressure equivalent to the weight 

 of 46,000 feet of strata, instead of the 14,000 which formed its 

 actual cover; but this conclusion, as has been said, must be applied 

 to the mineral, not the rock. It is improbable that the overlying 

 strata would be able thus to, withstand an upward pressure equal to 

 more than three times their weight. Mr. Ward shows that, as 

 regards this wide discrepancy, the Shap Fell rock is exceptional 

 among the Lake District granites ; but he fails to notice that it is 



