1889.] Contact M et amor phis m in Silurian Rocks. 



195 



andesites or dacites seem more common. Mr. Allport's collection, as 

 will be seen, represents the effects of contact metamorphism on 

 rocks generally not rich in lime, and varying from silty clays to 

 greywackes. • 



The minerals which have been produced by contact metamor- 

 phism, in the rocks now studied, do not require a lengthy descrip- 

 tion. In addition to what has been written in England, we are 

 indebted to Professor Rosenbusch, Prof essor Barrois, and other foreign 

 observers for some admirable studies.* Of these results, a very clear 

 and full snmmary, up to a recent date, has been given by Mr. Teall,f 

 so that it is needless to enter into details which may now be regarded 

 as matters of common knowledge. The minerals in these specimens 

 from Scotland are : — 



(1) Quartz, which is very abundant, occurring both in small 



granules and in fair-sized grains, generally rather con- 

 spicuously free from inclusions. 



(2) Mica, (a) brown, sometimes becoming greenish coloured, 



apparently from subsequent alteration, J (b) white. 



(3) Hornblende. Occurs in grains of rather irregular outline, and 



not seldom interrupted by enclosures of quartz ; sometimes 

 rather acicular in habit and grouped. The length varies 

 from about 0*01 " to 0'025". Colour in thin sections a rather 

 pale olive-green ; pleochroism not very strongly marked ; 

 a, pale yellowish-green, Ij, olive-green or a rather deeper 

 green. Some might certainly be called actinolite ; the rest 

 is nearer to one of the secondary hornblendes seen in 

 epidiorites. Hornblende does not appear to be very common 

 as a product of contact metamorphism in sedimentary rocks. 



(4) Augite. Occurs in two or three slides with hornblende ; in one 



case rather abundantly. In roundish or elongated grains of 

 granular texture, almost colourless, or with the faintest 

 possible tinge of greyish -green. Cleavage not very distinct, 

 but sometimes grains show pretty clearly a cleavage parallel 

 with ooP, and then extinguish at angles often greater than 

 30°. Tints with crossed JSTicols often brilliant. This mineral 

 in general appearance resembles sahlite or malacolite, which 

 are frequent contact products in limestone, so we may 

 venture to regard it as a variety of augite. 



* Lossen, ' Zeitsch. Deutsch. G-eol. Gesell.,' vol. 19, p. 509 ; 21, p. 281 ; 24, p 701 ; 

 Kayser, ibid., vol. 22, p. 103 ; Zirkel, ibid., p. 175 ; Fuchs, ' N. Jahrb. Mineral.' 

 (1870), p. 742 ; Barrois, ' Ann. Soc. G-eol. du Nord,' vol. 11, p. 103 ; Levy, ' Bull. 

 Soc. Geol. Trance,' ser. 3, vol. 9, p. 181; Hawes, 'Amer. Journ. Sci.,' vol. 21, 1881, 

 p. 21. 



f ' British Petrography,' Chapter xii. 



1 See below for some remarks on this mineral. 



