18 J. CLIFTON WARD ON THE GRANITIC, GRANITOID, AND 



more highly magnified, in order that comparisons may be made with 

 that of the altered slate in their neighbourhood. 



1. St.- John's Quartz Felsite and altered Skiddaw Slate. — In my 

 Memoir on the Keswick district I have described the microscopic 

 structure of the various stages of altered slate around the quartz 

 felsite, but no figures are given. The chief characteristics are the 

 following : — 



Specimens of very decided slate, but with evident signs of altera- 

 tion, show the following structure under a high power : — an irre- 

 gular and patchy network of a yellowish-green mineral, sometimes 

 crystallized in fan-shaped masses, faintly dichroic and more or less 

 fibrous in appearance; mingled with this apparently chloritic or 

 semi-micaceous mineral are an infinite number of minute quartz 

 grains, while dark particles both of magnetite and carbonaceous 

 matter are diffused throughout. 



In the next stage of alteration, represented in fig. 1, PL II., the 

 base has a structure which is between granular and felsitic, though 

 more closely allied to the latter. In this base are many crystalline 

 grains of quartz, much larger than any occurring in the previous 

 stage ; and imperfectly formed felspar crystals are here and there 

 apparent. The yellowish-green mineral frequently occurs in larger 

 portions, and puts on in many cases more the appearance of mica 

 than in the last example. The quartz contains exceedingly minute 

 liquid-cavities, occasional grains of magnetite, and small portions of 

 the greenish mineral. 



Lastly, the rock assumes its fully crystalline structure (fig. 2) : 

 the felsitic base contains numerous crystals of felspar, while those 

 of quartz are developed in greater abundance and are generally of 

 larger size than in the previous stage, the liquid- cavities being also 

 larger. Mica is distinctly apparent, generally of a greenish colour. 



2. Crag-Bridge Quartz Felsite. — In the specimens examined the 

 base of this rock appears somewhat intermediate between that of 

 altered Skiddaw Slate and one truly felsitic, being composed largely 

 of quartz in small grains, and a great quantity of the yellowish- 

 green chloritic mineral frequently crystallized in radiate and fan- 

 shaped groups. Crystallized felspar seems rare, while mica is 

 present in long greenish flakes, and crystals of quartz are here and 

 there porphyritically imbedded. Besides these minerals a little 

 hornblende may also be present. This microscopic character of the 

 rock agrees with what might be inferred from its lithological appear- 

 ance, being a less decided quartz felsite than that of St. John's Yale, 

 and more resembling some of the minor patches. 



3. Buttermere and Ennerdale Syenitic Granite, and altered Slcid~ 

 daw Slate. — Figs. 3 and 4 (PI. II.) represent the highly magnified 

 microscopic structure of specimens of altered Skiddaw Slate from Bed 

 Pike and from near Scale Force, respectively. As before, we have 

 the yellowish -green mineral in profusion, grains of quartz, some- 

 times assuming a more or less crystalline form, and particles of 

 magnetite, together, probably, with some carbonaceous matter. In 

 the example from near Scale Force the green mineral is frequently 



