W. M. Hutchings — The Origin of some Slates. 267 



in conjunction with the high specific gravity they may be safely 

 set clown as being that mineral. Barium sulphate is frequently 

 found as a deposit in colliery waters, and doubtless these crystals 

 were formed from the solutions circulating in the sediment. 



The quartz-grains average from toVo to ttmttt inch in diameter ; 

 they are in all respects of the nature of granite-quartz and are both 

 angular and more or less rounded. Some of them I consider are 

 corroded, but I confess I do not find myself able to make quite sure 

 of this, though Mr. Sorby says that chemically corroded quartz is 

 readily distinguished from simply worn grains. 



The numerous felspar-grains are more rounded than the quartz. 

 Many are pretty fresh ; but unless cleavages or other special marks 

 facilitate recognition, it is safer not to set down anything as 

 felspar which does not distinguish itself from quartz by its optic 

 figure in convergent polarized light. One or two grains of plagio- 

 clase were observed giving nearly straight extinctions, — presumably 

 oligoclase, and one or two grains of microcline. 



Mica, both muscovite and biotite, is abundant, the biotite being 

 in excess over muscovite. The largest flakes measure about tu"o inch 

 in longest diameter; thus one biotite flake measured T ^o- by s\o inch, 

 and there are all sizes down to very small indeed. The muscovite 

 is in all the distinct separate flakes seen in the sections, or isolated, 

 in all ways fresh and normal, and shows no inclosures of any sort 

 beyond those to be seen in granite, i.e. small zircons, etc. 



Of the biotite the greater part is more or less altered, but the 

 still fresh flakes show it to be optically and otherwise a quite normal 

 granitic type. 



It seems thus quite clear that in this deposit we have the waste 

 of a granite with two micas, and I am not able to detect any grains 

 of any sort to give rise to the inference that other rocks, older slates 

 for instance, took part in supplying the material. 



The alteration taking place in the biotite is of much interest for 

 reasons already alluded to. Contrary to what would be expected, 

 alteration to chlorite does not take place, or if it does, it is only to a 

 very slight extent. A very small number of greenish flakes may 

 be observed, but they are not chlorite. Such green flakes are seen 

 in granites and other rocks, and may mark an early stage of the 

 alteration of biotite to chlorite, though I believe they are in some 

 cases looked upon as an original greenish mica. They certainly are 

 not chlorite yet, and I have not detected any of that substance in 

 these deposits. 



The alteration which has taken place so extensively shows itself 

 in a bleaching of the biotite and the development in it, in large 

 quantity, of a substance of high refraction and very high double- 

 refraction. It appears to be mainly epidote, which mineral is well 

 known to be frequently produced during the alteration of biotite. 

 It forms in clusters of small round or more or less oval granules, in 

 irregular plates and flattened grains, and sometimes in plates of 

 more definite outline, among which a few may be found as large 

 as loVff inch in diameter, showing the form sometimes of four-sided 



