HEMITHRENES AND OTHER CALCITIC ROCKS. 57 



In our paper of 1868 we also noticed the occurrence of crys- 

 talloids of malacolite in ophite from Connemara, which are not 

 only more or less externally eroded and separated by calcite, 

 but have gaping or chink-like cleavage -partings filled with the 

 same substance. The mineralogist, it was asserted, knows full 

 well that originally the cleavage- par tings had individually their 

 two planes in perfect contact ; hence there is no other explana- 

 tion open to him than the one which admits that some solution, 

 charged with carbonic acid or a carbonate, has gained access into 

 the cleavage-partings of the malacolite, and, necessarily reacting 

 on some of its constituents (? calcium silicate) , has generated 

 the infilling of calcite. 



Reverting to our account of the different minerals in the 

 Tiree marble, it was stated that we had found in this rock 

 'crystalloids of hornblende, others of sahlite, a few of quartz, and 

 some apparently of serpentine ; while an occasional one appears 

 half composed of hornblende and the other half of sahlite "*. 



Pretty much the same result has been lately obtained by 

 Heddle, who mentions portions of sahlite in which " an inci- 

 pient as well as a perfected passage into serpentine is seen^f- 



Since the year 1869 we have described the microscopic struc 

 ture of hemithrenes from the Isle of Skye, New Jersey f, Aker, 

 and Ceylon. Zirkel, in 1870, noticed the " roundish, sharply- 

 defined, serpentine grains " in Scandinavian hemithrenes, and 

 assumed them to be pseudomorphs. In our papers it was shown 

 that they contain not only rounded crystalloids of malacolite and 

 white pargasite (or it may be wollastonite) , singly and in aggre- 

 gations, but the same things decreted and fashioned into slender 

 shapes, corresponding with the remarkable configurations already 

 described as occurring in Canadian ophite. A specimen from New 

 Jersey contains numerous configurations associated with spinel, 

 a well-developed example being actually imbedded in the calcite 

 occupying a fissure-like opening in a large octahedral crystal of 

 this mineral|| . Specimens of the Skye rock, where it is ophitic^, 



* Proc. Roy. Irish Academy, July 12, 1869. 



t Op. cit. p. 459. f Proc. Roy. Irish Acad. n. s. vol. i. (1871). 



Geological Magazine, vol. x. Jan. 1873. 

 || Proc, Roy. Irish Acad. vol. x. p. 547 (Feb. 1870). 



5[ This rock seems for the most part to be a carrarite j but the portions we 

 have examined are in the state of ophite or hemithrene. 



