Vol. 62. ,] BUTTERMERE AND ENNERDALE GRANOPHYRE. 259 



In the granitic forms, orthoclase, perthite, and plagioclase all 

 occur as large idiomorphic or hypidiomorphic crystals. The ortho- 

 clase and perthite are twinned on the Carlsbad law, and are of 

 quite normal character. In the granophyric forms also, orthoclase 

 occurs in graphic intergrowth with the quartz. 



In most examples the dominant felspar is a rather turbid plagio- 

 clase, which occurs in fairly well-formed crystals in nearly all 

 varieties. It is commonly twinned on the Carlsbad and albite- 

 laws ; the twin-lamellae are often rather indistinct, owing to 

 decomposition. Occasionally other twins appear, such as the 

 pericline and Baveno varieties. The plagioclase -crystals are some- 

 times slightly zoned, but the range of composition between the inner 

 and outer zones is not wide. Examination of a very large number of 

 symmetrical sections shows that, in the majority, the extinction is 

 nearly straight. Some examples show extinction-angles up to about 

 20° ; and in these cases a comparison of the index of refraction with 

 that of quartz proves that andesine and albite are both present. 



The analysis tabulated by Clifton Ward x is probably not very 

 trustworthy ; but, if we assume it to be moderately correct, the 

 percentages of soda and lime which are there stated, supposing all 

 to be combined as felspar, would give an approximate ratio of 

 Ab : An = 6 : 1. But some of the soda exists as perthite, and the 

 true ratio must be somewhat lower than this. 



The determination of the original character of the ferro- 

 magnesian mineral of this rock presents a certain amount of 

 difficulty. Clifton Ward regarded it as hornblende, while Dr. Teall, 

 in his ' British Petrography/ merely refers to it as ' green chloritic 

 aggregates.' 



In most specimens the coloured mineral is some variety of 

 chlorite, but in a few especially-fresh examples from well-glaciated 

 localities I have found almost unaltered biotite of a yellowish- 

 brown colour, and in the normal rock the chlorite undoubtedly 

 represents this mineral ; while, in the more basic modifications from 

 Bowness Knott, Seatallan, and elsewhere, it is equally clear that 

 much of the chlorite is derived from augite. 



The chlorite of the normal rock occurs in ragged flakes having 

 the form of biotite ; it is of a rather deep-green colour, and strongly 

 pleochroic (a deep green, c pale yellowish-brown). The extinction, 

 is straight, parallel to the cleavage of the original biotite. This 

 mineral is only distinguished from green biotite by its extremely- 

 weak double refraction ; sections of the ordinary thickness generally 

 give a steel-grey of the first order, while many are completely 

 isotropic. The flakes of chlorite very often enclose rather large 

 crystals of a mineral with high refractive index and strong double 

 refraction: this appears to be epidote. These inclusions are 

 surrounded by a well-marked dark 4 pleochroic halo ' [4763]. 



The characters of the green mineral seem to agree best with the 

 variety of chlorite known as pen nine, while a few sections 

 1 Quart. Journ. Geol. Soc. vol. xxxii (1876) p. 22. 



