Arthur Holmes — The Granophyres of Car rock Fell. 405 



detected, but there are a few vague dark-green to black chloritic 

 alteration products that may represent traces of a mafic mineral. 



Sections show the presence of two kinds of microscopic veins, both 

 being exceedingly thin and dying out within the limits of the section. 

 The first of these consists of a quartz mosaic containing tiny specks 

 of muscovite that twinkle like calcite over a rotating nicol. The 

 second type of vein consists of dolomite and calcite, and is, as far as 

 the evidence goes, later than the quartz type. The rock contains 

 sufficient carbonate to effervesce with cold dilute acid when 

 powdered. The analysis indicated that not all the carbonate could 

 be calcite, and to test this point an uncovered section was prepared 

 and treated with Lemberg's (logwood) solution. The greater part of 

 the carbonate mineral failed to take the stain, thus confirming the 

 presence of dolomite deduced from the analysis. 



The chief accessory mineral seen in thin section in addition to 

 those already mentioned is pyrite, though only two or three small 

 cubic cross-sections have been seen in several slides. However, 

 accessory minerals were also sought by the method adopted in the 

 investigation of other Lake District granitic works by Messrs. Kastall 

 and Wilcockson. 1 Some of the rock was crushed and a separation of 

 the heavier particles was effected straight away with a Thoulet 

 solution (potassium mercury iodide) having a specific gravity of 

 about 3. A preliminary separation by panning was avoided because 

 of shortage of material, and because of the paucity of heavy 

 minerals, most of which would have been washed away by such 

 a process. As soon as the crushed rock was stirred into the solution 

 a few black grains could be seen falling through the white felsic 

 constituents, and after a few minutes sufficient material was drawn 

 off for examination. The magnetic portion, which was about a third 

 of the whole, contained pyrrhotite with a little magnetite. This 

 result is interesting in view of the universal occurrence of pyrrhotite 

 in the other Lake District granitic rocks. 3 The residue (not picked 

 up by a bar-magnet) consisted, in order of abundance, of pyrite, 

 ilmenite, and zircon. 



Chemical Analysis. 



In calculating the mineral composition as given below, I have 

 departed from the conventions of the norm. The norm would have 

 shown figures for orthoclase and corundum. Corundum is certainly 

 not present and orthoclase has not been detected, its place being 

 taken by muscovite and sericite. For this reason the potash was 

 expressed mineralogically as muscovite and the remaining alumina 

 calculated as kaolin. As there is insufficient lime to satisfy all the 

 carbon dioxide (and therefore none available for felspar) it was 

 necessary to use some of the magnesia for this purpose, a procedure 

 justified by the actual presence of dolomite in the rock. "Hyper- 

 sthene " is probably the partial representation of the chloritic 

 alteration products mentioned below. 



1 Q.J.G.S., Ixxi, p. 592, 1915. 



2 Ibid., p. 617. 



