57 



Many of both adjacent rocks are found in an altered 

 state imbedded in the trap ; which, with the filtering of the 

 limestone, prove the deposition a true dyke. 



The mass of the rock consists of greenstone, sp. gravity 

 2.87, of a dark green, but frequently veined and mixed with 

 many other minerals. 



In the centre of the exposed portion of the dyke rises a 

 large vein of nearly white hornstone, presenting very inte- 

 resting characters. It contains no imbedded minerals, and 

 is homogeneous in structure, but with a lamellar or pseudo- 

 crystalline arrangement. Its planes are vertical, and at its 

 junction with the trap it is moulded to it, but not adherent, 

 and appears to have been formed from rocks at a greater 

 depth than the trap, and ejected through it. The minerals 

 found imbedded in this trap rock are various ; specimens 

 have been obtained of mica, chlorite, felspar, albite, olivine, 

 augite, amphibole, epidote, apatite, adularia, chalcedony, sul- 

 phate of lime, probably anhydrite, baryto-calcite, arragonite, 

 calcareous spar, fluor spar, galena, iron pyrites, sometimes 

 magnetic. Epidote is found also on Mutton Island. 



The general mass of this trap rock possesses a hidden 

 nodular structure, only developable by blasting. The no- 

 dules consist of precisely the same material as their matrix, 

 and having the same cohesion, they cannot be detached 

 by the, hammer. 



The nodules are from eighteen inches in diameter to the 

 size of a nut ; they are sometimes found pressed together in 

 masses with flat sides, like bubbles. Crystals occurring at 

 the surface of a nodule do not pass into the matrix, but are 

 truncated thereby. In some cases the nodular structure is 

 gradually obliterated, and the usual homogeneous one re- 

 places it. 



This nodular formation is essentially different from any 

 hitherto described, — as the orbicular granite of Corsica and 

 South of France, the onion stone of the causeway, &c.j 



