AND ON THE ORIGIN OF SOME EPIDOSITES. 741 



By transmitted light the decomposition-product appears to be 

 chiefly epidote with possibly a little kaolin. The latter would 

 naturally have resulted from the alteration of the f elspathic portion 

 of the felsite ; while the epidote seems, in great part, to have been 

 formed within the minute fissures and perlitic cracks by which the 

 rock is penetrated. Specks of haematite are also present. 



The prevailing greenish colour of the rock is due to the epidote, 

 of which the largest crystals occur chiefly in the quartz-veins. The 

 crystals are too small and, as a rule, too imperfectly developed for 

 the measurement of the oblique extinction-angle in clinopinacoidal 

 sections ; but in one case it was found to be about 28° to the edge 

 001 : 010, or trace of the basal plane, the angle between the faces 

 001 and 100 in this instance being 113° to 114°. If the section 

 were truly parallel to the clinopinacoid this angle should be 115° 

 24'. Sections in the orthodiagonal zone give parallel extinction. 

 The close massing of the epidote renders the perlitic structure some- 

 what obscure in transmitted light. The structure, however, appears 

 more distinct when surface-illumination is resorted to ; but, even in 

 transmitted light and between crossed nicols, the most decomposed 

 portions of the section may be seen to have a perlitic structure. 



Under these conditions of illumination this may be recognized by 

 any one or more of the following characters : — 1st, by the segregation 

 of epidote along the perlitic cracks being denoted by the interference- 

 colours ; 2ndly, by feeble transluceuce verging on opacity along the 

 cracks due mainly to the close massing of very small grains of epidote ; 

 3rdly, by delicate rings or curved lines of felsitic or simply siliceous 

 matter, the continuity of which is apparently broken by the ex- 

 tinction in any given position of certain of the minute crystalline 

 grains which constitute these delicate strings, so that an ever-shifting 

 succession of bright points may be traced along the perlitic cracks 

 during rotation of the section ; and 4thly, by oval or irregularly- 

 shaped nuclear residues of felsitic matter lying within the perlitic 

 areas, and free, or comparatively free, from epidote. 



In section 'No. 2, from which the accompanying drawings 

 (PI. XYII.), were made, the decomposition-products are not so 

 abundant as in No. 1. They are, however, present to a considerable 

 extent and also consist mainly of eiDidote, which, by its segregation 

 along irregular, wavy, and anastomosing lines, indicates that a 

 banded structure may also have existed in the rock. 



The specimens do not disintegrate at all readily when scraped on 

 a smoothly cut surface with the point of a penknife, and, indeed, 

 there is more knife than rock abraded during the process. It does 

 not therefore appear that kaolin is present in any appreciable 

 quantity, if at all, or the rock would be less coherent. We may 

 consequently assume that most, if not all, of the greenish-white 

 matter in the rock is epidote. It is, however, probable that kaolin 

 represented the flrst stage of decomposition of the felspathic consti- 

 tuent of the felsite, and that the epidote has resulted from the 

 alteration of this first decomposition-product by the action of water 

 charged with bicarbonate of Hme and more or less carbonate of iron 



