OF ARTS AND SCIENCES. 275 



amorphous, and in others contains minute fibres whicli polarize the 

 light very feebly. This soft amorphous substance is surrounded and 

 often ti'aversed by the associated oxide of iron. 



The plagioclase crystals, even where the twin striation in polarized 

 light is still preserved, contain a greater or less number of small 

 prehuite enclosures ; where these are more abundant, the twin striation 

 disappears, and in other parts of the field we find feldspar forms, pre- 

 senting now a brilliant aggregate polarization, with the characteristic 

 prehnite colors. More commonly the feldspar contours are lost, and 

 we find pseudo-amygdules of prehnite of all sizes, and with the most 

 irregular outlines. These often stand in such intimate connection with 

 prehnite aggregates adjoining feldspar crystals, that it is impossible not 

 to be convinced that the pseudo-amygdules are formed by a growth from 

 a central point outward, attacking the feldspar crystals bodily from the 

 outside. The pseudomorphs proper, on the other hand, result from 

 a change which had countless starting-points in the interior of the 

 crystal. Indeed, we have here a perfect counterpart to the formation 

 of chloritic pseudo-amygdules, and chlorite pseudomorphs after plagio- 

 clase, which 1 have described above, except that here we have a process 

 of prehnitization, while there it is one of chloritization. 



A remarkable feature in these sections is the almost total absence 

 of chloritic substaince ; and it is almost impossible not to see a connec- 

 tion in this with the absence of pyroxene. Indeed, it shows that it is 

 at least quite possible that the extensive change to chlorite in these 

 rocks may be preceded, or mediated, step by step, by the formation of 

 prehnite, which, in its turn, is changed to chlorite, under the co-operation 

 of the decomposition products of pyroxene. 



The paragenetic scheme for this rock may be written thus : — 



(I.) 1. Chrysolite. 2. Plagioclase. 



(II.) Pseudomorphs of soft green 



MINERAL <aild SPECULAR IRON 



after chrysolite. 



nij\ Prehnite in xiseiulomorplis 



after plagioclase. and in 

 pseudo-amygdules. 



Bed No. 94, — Specific gravity = 2.94. Coarsely crystalline. The 

 most prominent constituent, forming 40-50 per cent of the rock, is feld- 

 spar, in narrow, tabular crystals, which are often one inch long. As a 

 rule, the feldspar crystals show, on the cleavage planes, broad bands 

 free from twinning, except where two or three almost microscopic 

 plates of plagioclase are interposed, and a considerable number of the 



