40 ROCK-METAMORPHISM. 



of hornblende, from St. Gothard*. There is nothing by which 

 the eye can detect a difference between these specimens, except 

 colour t : both consist, for the most part, of bundles of slender 

 radiating prisms, with frayed-out or divided terminations and 

 a distant transverse cleavage ; and so strikingly alike are the 

 specimens as to make the one appear as if it were a facsimile 

 of the other. 



No clearer proof of the psendomorphosis of serpentine after 

 tremolite could be adduced; nor could a more decisive case 

 be brought forward showing a dioritic rock methylosed into 

 ophite J . 



Of late years there has been a growing disposition among 

 geologists to look favourably on the view that ophite, or its 

 essential component, serpentine, has originated from peridote 

 or from rocks (peridolytes) rich in this mineral. F. Sandberger, 

 G. Tschermak, Zirkel, and Bonney may be classed in this school. 

 It cannot be denied that the common occurrence at Snarum of 

 pseudomorphs of serpentine after peridote, and the frequent 

 association of the two minerals in other places, may be taken as 

 good evidence in favour of this view; but it would be just as 

 reasonable to assume that basalt, because it usually contains a 

 large proportion of peridote, was generated out of masses of this 

 mineral. 



Zirkel has noticed, in a precited memoir, the occurrence of 

 " small, roundish, sharply denned crystalloids of serpentine " in 

 the " crystalline limestone " or hemithrene of Aker and Sala in 

 Sweden, Snarum and Modum in Norway, Pargas in Lapland, 

 and Lough Derryclare in Connemara; and he maintains that 

 they are the product of alteration in peridote, this mineral often 

 being present in the crystalloids as a core. 



We shall endeavour to show in another Chapter that peridote 

 is as much a secondary product as serpentine, and that, on this 



* According to Damour (Bischof, Chem. and Phys. Geol. vol. ii. p. 348), 

 the St.-Gothard tremolite consists of Si0 2 58-07, MgO 24-46, CaO 12-99, FeO 

 1-82. 



t In some instances the prisms retain their original colour. 



} Mr. Frank Rutley, who takes this to he a serpentinous rock, states that 

 some of the long radiating crystals it contains a display magnificent variega- 

 tions of colour under polarized light " (' The Study of .Rocks/ p. 131). Sections 

 examined hy us show nothing more than the colours of ordinary serpentine : 

 prohably Mr. Rutley's case contains perjdotic matter, 



