﻿170 PEOr. T. G. BONNET ON ANTIGORITE. [May I908, 



Still, I think them indicative of actinolite or partly-altered residual 

 pyroxene. As we find only very slight traces of any ferriferous 

 constituent, I would suggest the possibility of the rock having 

 originally consisted almost entirely of an augite akin to diopside 

 (like some which occur in Canada) rather than, as my late friend 

 suggested, an oli vine-rock from which the iron has been leached 

 out. 



Discussion. 



Dr. J. S. Flett congratulated the Author on having traced the 

 antigorite-serpentine of Yal Antigorio to its source. These rocks 

 were full of interest, though rare. Very few examples occurred in 

 the British Isles : the speaker had met with some in Perthshire 

 accompanying the great epidiorites that go with the Loch-Tay 

 Limestone ; he had also seen specimens which were believed to come 

 from Shetland, and were as perfect as any that the Author of the 

 paper had exhibited on this occasion. In Unst and Petlar, in the 

 extreme north of Scotland, there were large masses of serpentine 

 known to geologists through the descriptions of Hibbert, Heddle, 

 Peach, and Home. In Balta Sound and Haroldswick the Unst 

 serpentine was intersected by many veins of pyroxenite. The whole 

 complex had been sheared, and in these veins antigorite was 

 developing from the pyroxene, which corroborated the Autlior's 

 views as to the origin of this mineral. 



Mr. J. Allan Thomson instanced a case of pseudomorphism of 

 olivine by an antigorite-like mineral in the hornblende-peridotite 

 of Greystones. It differed from true antigorite in possessing a 

 very faint pleochroism and higher birefringence. The Author had 

 concluded in his former paper that these properties, which he 

 had sometimes found in minerals accompanying antigorite, were 

 accidental. They might, however, be explained on Prof. Tschermak's 

 hj'pothesis of the composition of the orthochlorites as solid solutions 

 of the antigorite- and amesite-molecules. Dr. Weinschenk con- 

 sidered iddingsite as a pleochroic antigorite. 



The interesting point about the pseudomorph was that the 

 antigorite-flakes lay parallel to the clinodomes of the olivine, which 

 met at an angle of about 60°. The resulting ' gitter '-structure 

 might easily be confused with the lattice-structure of serpentine 

 derived from a non-aluminous amphibole. Dr. AYeinschenk had 

 described the same phenomena in the olivine of the Stubach serpen- 

 tine, and considered the antigorite as an original mineral intergrown 

 with olivine by ' piezocrystallization.' 



The Author said that he was much interested to hear from 

 Dr. Flett that antigorite-serpentines had been found in Shetland, 

 and should look forward to reading the account of them. The 

 replacement of olivine by antigorite, in the case described by 

 Mr. Thomson, was remarkable ; but he thought that any definite 

 relation between the flakes of the latter and the crystal-figure of 

 the former mineral was very rare. He thanked the Fellows 

 present for having listened so patiently to a statement which had 

 taken more time than he had anticipated. 



