OP THE 80UTE OF THE ISLE OF MAN. 430 



on the north slope of Cadcr Idris, in macroscopical appearance. It 

 is a green, distinctly crystalline rock with specks of iron-pyrites. 

 Sp. grav. 2-87. 



As regards its microscopic characters *, I may mention that large 

 idiomorphic crystals of colourless (?) aiigite t, mostly altered to 

 brownish decomposition-products, are accompanied by a small 

 quantity of partially idiomorphic hornblende, brown by transmitted 

 light, and cloudy altered felspar containing apatite needles. The 

 space between the felspars is occupied by chlorite, calcite, and other 

 alteration-products. Small patches of leucoxene occur, and the rock 

 as a whole is much altered. 



(b) The Micro-ffranite Dylce. — Specimen 204 from centre of dyke. 

 A greyish fine-grained rock (coarse-grained for a micro-granite) in 

 which white mica in small hexagonal plates is more conspicuous 

 than dark mica ; with care porphyritic felspar-Crystals can be made 

 out, and the bulk of the rock is seen, on examination with a lens, 

 to consist of felspar and quartz. No parallel structure is visible. 

 The specific gravity of the rock is 2-62. 



Specimen 205 a, from about 6 inches from the western salband of 

 the dyke. In this a very distinct parallel structure or foliation is 

 developed, the foliation planes being parallel to the edge of the dyke. 

 The hexagonal plates of white mica are larger and more con- 

 spicuous than in 204, and have often their basal pinacoid faces lying 

 quite obliquely to the foliation planes, the direction of which is in- 

 dicated by fine dark subparallel lines traversing the compact light 

 grey groundmass, in which, by careful examination with a lens, 

 small porphyritic quartz-crystals can be made out. 



The following chemical analysis of a specimen of the Crosby dyke 

 close to the contact is given by Messrs. Dickson and Holland % : — 



SiO, 74-39 



Al,03 15-55 



Pe;03 1-35 



MnO 0-22 



CaO 0-48 



MgO 0-33 



K,0 2-14 



Na,0 3-79 



Combined Water. ... 1*18 



99-43 

 Sp. grav. 2'72(?). A specimen lent to me by Mr. Dickson gave 

 2-62. 



The rock is therefore a micro-granitic keratophyre (soda-felsite). 

 If the whole of the potash be calculated as orthoclase from the 



* Compare remarks on sect. 250 among Supplementary Observations. 



t If it were not for the high extinction-angle (about 40°), I should be dis- 

 posed to regard this as epidote pseudomorphous after hornblende. 



\ Proc. L'pool Geol. Soc. vol. vi. pt. i. (1889) p. 126. Compare the analyses 

 (and descriptions) of soda-felsites from Co. Wicklow in Dr. Hatch's paper, 

 Geol. Mag. (1889) p. 546. 



