W. M. HatcJiings — Ash-slates of the Lake-District. 227 



Under these circumstances it has not been possible to obtain any 

 very effective chemical evidence on the point in question. 



The most favourable rock in my possession is an andesite from 

 a point not far from Ulles water, on the road to Matterdale. Its 

 ground-mass is fine-grained, and appears to have been originally 

 *' hyalo-pilitic." There are numerous well-defined pseudomorphs of 

 chlorite after augite. The porphyritic felspars are numerous, but 

 small, ranging from about aVth of an inch in length as maximum. 

 They are largely more or less glassy clear, and contain comparatively 

 little of secondary minerals ; — chlorite, calcite, a little epidote, with 

 but little mica in this case. 



A good many of them show the appearances above stated which 

 lead me to suppose they are perhaps orthoclase. The rest are 

 plagioclase, of which, as usual, it is not possible to safely determine 

 the variety, though oligoclase appears to be present. A large piece 

 of this rock was pulverized and reduced to uniform very small 

 grains by means of wire gauzes of suitable mesh. The powder was 

 subjected to repeated separations in liquids of diminishing specific 

 gravities, the lighter portions being successively concentrated, a 

 final separation being made in a liquid in which a fragment of pure 

 labradorite just remained suspended. In this manner, with plenty 

 of time and patience, it was possible, in spite of the unfavourable 

 nature of the rock and of the smaliness of the grains, to separate 

 from a large bulk a small quantity of material which proved to be 

 felspar, free from ground-mass, and contaminated only by small 

 amounts of chlorite and calcite. This was finely powdered, digested 

 a short time in hydrochloric acid of moderate strength (sufficient to 

 decompose chlorite), and finally in dilute potash liquor to remove 

 separated silica. The residue, well washed and dried, was practically 

 pure felspar. 



A sample of the bulk of the powdered rock contained : Silica, 

 62-43 per cent. ; Soda, 4'13 per cent. ; Potash, 2*28 per cent., as 

 kindly determined for me by Dr. J. B. Cohen. This does not differ 

 appreciably from the average run of analyses of local andesites, 

 though the potash is perhaps a trifle higher than usual. 



The isolated felspar-substance was analyzed by Mr. Paterson and 

 contains Soda 8-52 p.c. ; Potash 4-1 p.c. 



Having regard to the facts that in all rocks of this class the 

 potash concentrates in the ground-mass, and that the average of the 

 entire rock is here only 2-28 p.c, we should not expect to find 

 anything like so much as 4 p.c. potash in the porphyritic felspars. 

 Its presence in them in this amount would seem to justify the in- 

 ference that some of them are specially rich in it, and that, indeed, 

 some potash-felspar is probably present together with the plagioclase. 



In some of these rocks, again, there appear to be examples of 

 those very interesting phenomena which have been so beautifully 

 described and illustrated by Prof. Judd in his paper, " On tlie 

 Growth of Crystals in Igneous Rocks after their Consolidation " 

 (Q.J.G.S. voL xlv. 1889). 



They are not so large nor so striking as the instances quoted and 



