50 EOCK-METAMORPJHISM. 



Examined by the microscope, the green layers are seen to 

 consist of flattened radiating tufts of tremolite and long crystals 

 of hornblende, both tufts and crystals lying with their long axis 

 parallel to the lamination. Intermixed with these minerals are 

 peridote (beautiful in its colouring), serpentine in grains and 

 flattened lumps, a variety of the latter (apparently pyrosclerite) 

 forming veinlets, and chlorite in small quantity; the latter 

 mineral, however, is abundant in some specimens, associated 

 with serpentine. The pale brown layers consist of malacolite, 

 a striped feldspar, and calcite. The malacolite, translucent and 

 opaque, is in different-sized crystalloids confusedly aggregated : 

 those that are opaque have usually a flocculent coat; while 

 their angles and edges are more or less rounded, giving rise to 

 interspaces, which are filled with calcite : occasionally the calcite 

 increases to such an extent as to form thin layers, deviously di- 

 viding those of the malacolite ; and where this happens the crys- 

 talloids of the latter mineral are greatly eroded and reduced in 

 size. The calcite is decidedly more abundant in these layers 

 than in those consisting of tremolite &c. Grains and granular 

 aggregations of serpentine, occasionally intermixed with tremo- 

 lite, are also present in the former. ....;>" L ..:. 



Thus the Glassillaun rock is an intimate mixture of mineral 

 silicates and a mineral carbonate ; and as such it must be con- 

 sidered to be a true hemithrene. 



Mr. Twining' s observations, kindly made at our desire, make 

 the promontory 200 feet in width, and projecting 40 feet into 

 the water at high tide. The hemithrene occupying the ditch- 

 like channel becomes on the west side of the promontory a 

 more dilated mass, much of which, except at the edge of the cliff, 

 is obscured by a growth of seaweed, boulders, and beach- sand. 



An opportunity will occur hereafter, enabling us to account 

 1'or the presence of peridote in methylosed rocks; we may 

 therefore confine ourselves to the serpentine and Calcite of 

 the hemithrene in this instance. The serpentine is so associated 

 with the tremolite as to make it evident that the former has 

 resulted from chemical changes effected in the latter ; and there 

 is nothing more certain than that the calcite stands in the same 

 relation to the malacolite. 



That water .has accompanied the changes is proved by the 

 presence of serpentine in the hemithrene; and the fact that 



