478 I'n.f. T. G. Bouncy an.l .Miss Q A. Raisin. 



a gradual replacement of portions of the ground-mass, so as to form 

 ultimately a kind of setting for the grains which remain. In other 

 parts, however, though a considerable portion of the ground-mass 

 persists, the characteristic cleavage of the mica can be readily 

 detected,* its pleochroism being wanting, while a straight extinction 

 is quite discernible. In these cases we have, as it were, the ghost of 

 the mica, but commonly, as the mineral becomes more and more 

 characteristic and pleochroic, the constituents of the ground-mass 

 correspondingly disappear, until at last only few of them remain 

 (fig. 2). In these, however, the original orientation is still preserved. 

 Towards the edge of some of the grains the white mica, chlorite, <fec., 

 of the ground-mass seem, as it were, to pierce the brown mica. It is 

 quite clear that this mineral has been formed after the production of 

 the cleavage-foliation in the rock.f The manner of its occurrence 

 suggests very strongly that its composition differs but sliirhtly 

 (except for the absence of C0 2 ) from that of an average sample of 

 the ground-mass. 



We are indebted to Mr. P. Williams for the following analysis of this 

 mica, made in Professor Ramsay's laboratory at University College. 

 As only a very small amount of the mineral could be spared for the 

 purpose, he was placed under considerable difficulties, and found it 

 necessary to compute the alkalies as potash. It must be also re- 

 membered that the crystals are rarely quite free from particles of 

 ground -mass : 



102-2 



The analysis corresponds generally with that of a hydrous mica, 

 but has more potash (or alkalies) than is usual. It differs from 

 biotite, which the mineral most resembles, in the higher percentage 

 of alumina (in which it comes nearer to mnscovite), and in the large 

 amount of lime. So far as the mineral can be classified it appears to 

 be a hydrous biotite, with a considerable part of the magnesia re- 



* The cleavage locally is so strongly marked that at first sight one almost antici- 

 pates a twinning. The cleavage eeeins to be present, if one might say it, almost in 

 advance of the mica. 



t See T. G. Bonney, ' Quart. JL Geol. Soc.,' 1893, vol. 49, pp. 104113, fig. 1, 

 p. 107. 



