12 KOCK-METAMORPHISM. 



CHAPTEE IV, 



ORIGIN OF CERTAIN MINERAL, STRUCTURAL, AND 



CHEMICAL CHARACTERS OF OPHITES AND 



RELATED ROCKS. 



THIN sections of peridote from Elfdalen, in Dalecarlia, exhibit 

 this mineral in an amorphous or finely granular condition, and 

 not unfrequently containing thin fibrous or striated undulating 

 laminae, more or less separated and subparallel : the striae or 

 fibres are at right angles to the surfaces of the laminae (Plate I. 

 fig. 3). 



A similar structural character, seen on one of the two sets of 

 eminent cleavage-planes, marks the feldspar exhibited in fig. 4, 

 Plate I. 



From Harris (Hebrides) we have a remarkable quartzose 

 feldspathic rock, called " graphic granite" (see Plate I. fig. 5): 

 its feldspar, of a white colour, is in layers (a) transversely inter- 

 sected by striated laminae (a), which alternate with others (b) 

 devoid of structure, except occasionally a platy kind. The strise, 

 which lie at right angles to the boundary-planes of the laminae, 

 are of the kind called ' ' striping," characteristic of triclinic feld- 

 spars'*. 



This graphic granite also contains layers of quartz (b) alter- 

 nating with those of the feldspar, the latter being the thickest. 

 The striated laminae contained in the feldspar layers meet those 

 composed of quartz at a few degrees from a right angle : in the 

 example figured one of the striated laminae abruptly terminates 

 against a quartz layer. 



These three cases, with others that could be adduced, offer a 

 striking similarity to the amorphous and fibrous interlamina- 

 tions of the Colafirth ophite. The fibrosity, or call it striation, 



* We have studied in this connexion the striping of. feldspars, but without 

 arriving at any satisfactory conclusion as to its nature, save that it may be 

 related to striation and fibrosity. Looking at the fibrous cleavage charac- 

 teristic of selenite, possibly we have in these different structures modifications 

 of typical mineral cleavage : the stylolitic structure of rocks seems to be 

 another form, 



