38 ROCK-METAMORPHISM. 



published some important and reliable details on the pheno- 

 mena in question in his fourth chapter on " the Mineralogy of 

 Scotland/' thus expresses himself: c< All the serpentines of 

 Scotland which I have had opportunities of properly studying 

 are metamorphic rocks, formed for the most part by a change 

 of augitic and hornblendic rocks as diallage, euphotide, and 

 diorite. 



"The serpentines of Unst in Shetland are derived from 

 diallage. Of the two beds to the west of Portsoy, the first 

 from gabbro, the latter apparently from euphotide ; the beds to 

 the east, from a rock chiefly augitic. 



"The peculiar structure of the serpentine of the hill of 

 Towanrieff would lead to the conclusion that gneiss was the 

 original ; but the nearest rock is a laminated diorite, composed 

 of labradorite and black mica. Though these conclusions are 

 chiefly the result of geognostic observations of the district, 

 there are many localities where the transition may be traced 

 through a gradual change in the minerals composing the rock. 

 Such comparatively molecular transformation may be well stu- 

 died on the north shore of Swinaness, in Unst, in several places 

 in the neighbourhood of Portsoy, on the north side of the hill 

 of Towanrieff, and on the northern slopes of the Green Hill of 

 Strathdon. At the last-mentioned locality there may be ob- 

 tained unaltered, or apparently unaltered, diorite ; the same 

 with the hornblende duller in lustre and softer than normal, 

 and the felspar dull, semi-opaque, and of a greasy lustre ; and 

 lastly, almost perfectly formed serpentine, in which, however, 

 the granular structure of the altered rock is plainly visible. 

 These three occur within the space of a few feet of each other. 

 It is not, however, easy to select for analysis, from rocks the 

 several crypto-crystalline ingredients of which give way to the 

 transmuting agent at different periods of time specimens at once 

 typical and sufficiently pure. I have met with more success 

 in this direction in working among the serpentinous marbles 

 those which contain imbedded granules or patches of serpen- 

 tine than I have among the larger masses of the serpentine 

 rock itself. 



" One fact I would direct attention to, seeing that it has 

 perhaps not been clearly enough considered, namely, that great 

 beds of serpentine must have been formed by the metamorphism 



