ASSOCIATED KOCKS OF THE LIZAED DISTRICT. 921 



parallel to the junction-face of the rock ; and I regard it, like that 

 in the Karak-Ciews gabbro, as a pressure rather than a flow struc- 

 ture.* On applying the two Nicols, the major part of the slide is 

 seen to consist of the usual bluish-grey fibrous mineral ; and the 

 only difference between that which makes up the mass of the rock 

 and that in the pale golden band is the greater regularity of crystal- 

 lization in the latter. A few minute crystals show a more brilliant 

 tint. 



Cam Sjparnack (no. 11). — Obviously an altered olivine rock, though 

 but little of the olivine remains unchanged ; the general aspect of 

 the groundmass resembles no. 3, except that there is no tendency 

 to a parallel structure, the irregular network of fine serpentine 

 strings with the granules bordered within by iron peroxide being very 

 distinct. The olive-green mineral, with submetallic lustre, which 

 forms the green marbling, is all or almost all enstatite, though here 

 and there probably is a little diallage ; but alterations and staining 

 make it difficult to be sure. 



Blade Serpentine from near Cadgiviili (no. 10). — The ground of 

 the slide is a pale green serpentine, traversed by numerous strings 

 forming the usual network ; these are colourless, except that they 

 are often darkened by the abundant deposit of fine magnetite dust. 

 Several small grains of picotite occur, and a good many crystals of 

 rounded exterior and platy structure — the same which is so con- 

 spicuous to the eye by its bright metallic lustre. The rock is clearly 

 an altered olivine rock; and on applying polarized light we see 

 that a few grains of that mineral yet remain unchanged. The 

 metallic mineral is all or almost all enstatite, though part of it is 

 rather altered, and converted into a kind of serpentine ; and on 

 comparing some of the specimens with that which I have described 

 in lherzolite from Sem (Ariege), one cannot fail to be struck with 

 the correspondence. f 



The Balk Serpentine (no. 9, two varieties). — These have the same 

 general character as the Carn-Sparnack rock, both being mottled with 

 olive-green ; but the ground of the one is more or less red, of the 

 other rather pale claret-colour. A detailed description is needless; 

 both obviously are altered olivine rock, the chief distinction being that 

 in the former the iron occurs rather in little patches and lines, often 

 fringing the included grains (which are clear but isotropic, and so 

 serpentine also), and is the red peroxide ; while in the other it is 

 more generally disseminated, giving a dusty look to the slide, and 

 appears to be mostly the black oxide, while the grains are generally 

 a dull pale green : both contain some picotite. The green mineral 



* It is obvious that very similar structures may result from tension and pre^ 

 sure ; thus the fissile structure of certain igneous rocks (as I have shown in 

 Quart. Journ. Greol. Soc. xxxii. p. 140) is like true cleavage. Professor J. D, 

 Forbes regarded the banded structure of glacier-ice as the result of differential 

 motion, like that of slags. Professor Tyndall considers it a pressure structure ; 

 while in some igneous rocks a pseudo -foliation seems to result, now from pres- 

 sure, now from tension. 



f See Mr. Hudleston's analysis of this rock, p. 925. 



