912 



T. G. BONNET ON THE SERPENTINE AND 



projecting, as happens with the gabbro of the Cuchullin Hills, Skye. 

 I have had slides cut from one of the dark bluish unweathered 

 specimens, and from one of the exterior white and green. The 

 former shows the rock to be an olivine gabbro. There is plagio- 

 clase felspar, generally in rather short irregularly oblong crystals, 

 showing bright colours and twinning. Some of these exhibit a pecu- 

 liar strongly marked cleavage (or minute twinning, with one set of 

 crystals dominating), which gives them a general resemblance to the 

 structure of diallage. The felspar is in places rather decomposed. 

 There is a fair amount of diallage, and a few crystals of common 

 augite. These with ordinary light are as nearly as possible colour- 

 less, and are in good preservation. The olivine is rather rough in 

 texture and much cracked ; the cracks are marked out by a deposit 

 of granular opaque mineral, probably magnetite, which in some 

 cases appears to penetrate the intermediate spaces (which are 

 commonly fairly translucent), rendering them almost opaque ; now 

 and then it assumes a browner tinge, as from haematite or limonite, 

 and the grains are slightly stained with brown or green. Most of 

 the olivine grains have a finely granulated aspect at the edges, and 

 are sometimes bordered by a finely fibrous mineral, probably ser- 

 pen tinous and a secondary product ; the grains, however, show 

 very little trace of conversion into serpentine. Except the minute 

 granules described above, there is very little magnetite or other 

 iron- oxide visible. The above appearance would lead us to conclude 

 that the olivine is a rather ferruginous variety. The other slide (fig. 7), 



Fig. 7. — Diallage partly altered into Hornblende, from outer part 

 of the great Gabbro mass at Coverack. 



A. Decomposing felspar. B. Diallage. C. Hornblende. 



cut from a somewhat weathered mass, which in appearance closely 

 resembled that of the veins, exhibits plagioclase felspar beginning 

 to pass into the saussuritic mineral, and diallage, more or less con- 

 verted into minute hornblende, but no olivine or serpentine that can 

 be recognized — the slide on the whole being remarkably like one 

 cut from a vein on the shore at Coverack. On closely examining 



