in certain Trap Rocks. 77 



Olivine, — massive, and in small crystals. 



Augite, Amphibole, Epidote, — in fine crystals; the latter also found on 

 Mutton Island. , . 



Apatite, — in microscopic crystals. 

 Chlorite, — indurated. 



Adularia, Chalcedony, — met with only in small specimens. 

 Sulphate of Lime, — probably "anhydrite." 

 Baryto-calcite, — crystallized in large masses. 

 Arragonite. 

 Calcareous Spar. 



Fluor Spar, — in purple cubic crystals, along with 

 Galena, — in cubes. 

 Iron Pyrites, — sometimes magnetic. 

 Specular Iron Ore, — in minute octahedral crystals. 



A i^vi of these minerals are rare in any habitat, others rare as occurring in 

 trap rock. Of these fluor has hitherto been found in trap only in two instances, 

 — at Gourock, in the Frith of Clyde, and at Papa Stour, one of the Shetlands. 

 Anhydrous sulphate of lime has been found in a trap-dyke at Cave Hill, near Belfast. 



Epidote is found in unusually fine crystals, imbedded in a red ochrey clay 

 (apparently of decomposed trap) on Mutton Island, in Galway Bay, the site of the 

 light-house. The island is low, and consists of sienite and trap, with a shingle 

 beach of those rocks and limestone pebbles intermixed ; epidote being also found 

 in rolled pieces. 



The iron pyrites is massive, and in crystals more or less perfect. The crystals 

 appear to be hexagonal prisms, always imbedded, and sometimes magnetic. 



By far the most singular circumstance, however, connected with this dyke, is 

 the fact, that it possesses very generally a hidden nodular structure, of a descrip- 

 tion different from that hitherto found in any rock. 



When a mass of this rock is separated by the hammer, it always breaks with 

 the kind of fracture before described, and no trace of any thing unusual can be 

 found. But in the process of blasting, the lines of least resistance in the mass 

 seem to be discovered by the expansive force with a beautiful precision, and the 

 fragments are found consisting of single or cohering nodules of trap, of various 



