I 



Vol. 68.] PETROLOGICAL NOTES ON GUERNSEY, ETC. 51 



of Eocqiiaine Castle is really only a case of extreme crushing/ It 

 is perhaps needless to add that the large area occupied by, and the 

 general uniformity of, the Pleinmont rock, make this explanation 

 impossible for it, so that Guernsey does contain one very ancient 

 sedimentary rock. 



VIII. J^OTES ON SOME PaKAMOEPHIC AND OTHEE ChANGES. 



[T. G. B.] 



The following alterations in minerals which have been mentioned 

 incidentally in the foregoing description deserve a little further 

 notice : — 



(a) The conversion of augite into hornblende. — I 

 •called attention to this change in 1877,^ and have since then 

 recorded other instances.^ In the case of uralite, the change 

 observed eighty years ago * was strictly paramorphic ; but, as the 

 term ' uralitization ' has been applied by modern writers in an 

 extended sense, it may be worth recording the several forms that 

 have come under my observation. Beginning with an augite (or 

 diallage) which in thin sections is almost colourless, we find that 

 it changes (1) into a rich brown hornblende, with no alteration in 

 external form. (2) This may be changed into a green hornblende, 

 which, however, may perhaps sometimes form directly from the 

 augite. (3) The latter mineral may be replaced by a group, the 

 •outline of which does not correspond with the original one, of small 

 hornblendes, either prismatic or acicular in form. The latter phase 

 may be locally subsequent to otie of the others, pale-green actino- 

 litic needles forming like a fringe at the margin of the larger 

 <irystals (4) ; but, when the whole of the new mineral assumes this 

 form, often becoming rather coarser in structure, that is probably 

 a result of pressure-metamorphism. 



The change from an almost colourless to a richly-tinted mineral 

 is rather surprising, especially in the first case. Probably some 

 ultra-microscopic iron-salt is developed,"' from which by further 

 action of water a hydrous iron-silicate (green) is formed. Stage (3) 

 probably indicates some irregularity in the operation of the trans- 

 forming agent, and (4) a greater activity of water. But, though 

 hornblende is often a result of change, paramorphic or otherwise, 

 in augite, we cannot doubt that in many cases it has been an 

 .original constituent of a rock. 



(/3) The conversion of hornblende into biotite. — In 

 1892 we described at some length cases of this change, as a con- 



1 The changes which pressure can produce in a gneiss are extraordinai'y ; 

 see Geol. Mag. dec. 2, vol. x (1883) pp. 509-11. 



^ Q. J. a. S. vol. xxsiii (1877) pp. 895, 909, 912, 915. 



^ As in Min. Mag. vol, ii (1879) p. 5. 



* By G. Rose, Pogg. Ann. vol. xxii (1831) p. 321. 



5 Perhaps a slightly hydrous iron-oxide, for that (with a little manganese) 

 is the main constituent of umber-brown tints. 



e2 



