No. 3.] MACFARLANE — ON CRYSTALLINE ROCKS. 259 



Opluocoma. 



FiMgments of a small species of ophiuroid starfish not deter- 

 minable, have beea found in the Leda clay at Montreal, and in 

 nodules at Green's creek. 



(2) Ecliinoidea. 



Earyecliinus drohachlensis, Miiller. 



Fossil — Leda clay, Beauport ; E-iviere-du-Loup ; Montreal. 

 This species is rare in the Post -pliocene, but very conimou in 

 all parts of the Gulf of St. Lawrence at present. 



(3) Hohtliuridea, 



Psolus phantopus '? Oken. 



Scales of an animal of this kind have been found in the Leda 

 clay at Montreil. They may beloni; to P. phanfopus, or to the 

 spscies P. (^Lopliothuria') Fabricii, also found on our coasts. 



ON THE ORIGIN AND CLASSIFICATION OF 

 ORIGINAL OR CRYSTALLINE ROCKS. 



By Thomas Macfarlake. 

 (^Continued from page 312 — Vol. V.) 



V. — MINERALOGICAL CONSTITUTION. 



Having*, in the forcgoinir, adverted to the texture and cheniicul 

 composition of original rocks, it now becomes necessary to refer 

 more particularly to their mineralogical constitution. In order to 

 continue the analogy which has been shewn to exist between fur- 

 nace sla2:s and oriuinal rocks, it will b3 well here to refer to those 

 instances which have boen observed of the formation of well 

 developed crystals in the cooling of artificial silicates. The rapid 

 manner in which furnace slags are commonly allowed to cool is of 

 course detrimental to the formation of any mineral-like aggrega- 

 tions, but it is sometinijs possible to observe in copper furnace 

 slags that, w^hen they have bi3n allowed to solidify in large blocks 

 or cakes, they shew an actynolitic structure in their mass, often 

 closely resemble hornblende rock, and very commonly contain 

 cavities lined with the most beautiful crystals. The formation of 

 pyroxene in slags from iron furnaces has been frequently observed 



