118 Proceedings of the B.oy(U Irifih Academy. 



usually is developed in small pockets, distributed regularly and abun- 

 dantly through the mass. As the mica decomposes more readily than 

 the other constituents, weathered surfaces of these mico-hornblende- 

 rocks have peculiar appearances. If the mica occurs in pockets or 

 nests, the rock surface becomes coarsely pitted (see fig. I. PL 9), while if 

 it is developed in large flakes, the rock when weathered presents sur- 

 faces on which are rude characters ha\dng a more or less angular arrange- 

 ment, as represented in the accompanying sketch map (fig. J, PI. 9) 

 of a portion of a rock containing large flakes of silvery white mica. 

 Such varieties of the hornblende-rock are easily recognised on account 

 of the peculiar weatherings. 



The varieties of hornblende-rock in places graduate into a granite 

 similar to the rock called Galway-type yranite in the memoirs of the 

 Irish branch of the Geological Survey. Its description will be given 

 hereafter. 



Actinolite or tremolite, wholly or in part, may replace the other 

 amphiboles, and the rock may consist of numerous hyaline acicular 

 crystals mixed with a felspathic paste, or numerous nests of such crys- 

 tals may occur in an ampbibolic-felspathic matrix. 



The actinolitic and tremolitic-hornblende-rocks are more or less 

 ophitic, or graduate into opkyte, steatyte, and such like rocks, the gra- 

 dation being very apparent among the rocks of the previously men- 

 tioned Cannaver Island. 



The ophyte of this area, due to the decomposition of igneous 

 rock, is usually of a dark leek-green, approaching black, colour', never- 

 theless a few subordinate patches of a light green were noted ; allvays 

 it is more or less associated with hornblende-rock in places having sub- 

 ordinate beds or layers interstratified ; the passage-rock between the 

 two may be called ophitic hor^ihlende-rock or amphiholic-ophyte, according 

 to the mineral predominating. In some of these passage-rocks crystals 

 of amphibole occur in an ophitic matrix, or blotches or maculse of 

 ophyte may be developed in an amphibolic base, or ophitic and amphi- 

 bolic particles may be more or less intimately mixed together. In some 

 it is evident that a hornblcndic mineral has produced the ophyte, while 

 in other rocks it seems to be a pseudomorphism of a felspar. In some 

 of these rocks, flakes of a mineral, in aspect like diallage, were 

 observed. 



Massive ophyte is di\T.ded by Dana into precious and common. The 

 first "is of a rich oil-grccn colour, of pale or dark shades, and translu- 

 cent even when in thick pieces." The second " is of dark shades of 

 colour, and sub translucent. The former has a hardness of 2"5 — 3 ; the 

 latter often of 4, or beyond, owing to impurities." In Avest Galway, 

 among the rocks of intrusive origin, no precious or )toble serpentine was 

 observed. 



A rock allied to those just described, and evidently a psouclomorph 

 of hornblende-rock, has a bright green colour, and an unctious, com- 

 pact, homogeneous, tough matrix, in which are imbedded more or less 

 irregular crystals or crystalline patches of garnet, flakes of mica, with 



