54 TIREY. MINERALS. 



it is the coccolite of mineralogists. The crystals are 

 occasionally defined ; but most commonly they are either 

 partially rounded or entirely shapeless, forming irregular 

 grains sparingly dispersed or accumulated in larger or 

 smaller groups. These accumulations are sometimes 

 very considerable in size, and admit of being detached 

 in large and well characterized specimens, the grains 

 varying much in colour, being black, dark bottle-green, 

 blueish green, and pale grey. 



The sahlite presents many varieties both of form and 

 colour, and is either compact or crystallized. In colour 

 it is snow white, and opake, or white and glassy ; from 

 which it passes into several shades of grey and green, 

 and more rarely into pale sky-blue ; presenting several 

 intermediate gradations from opacity to absolute tran- 

 sparency. The crystals appear under different forms. 

 The most perfect is a slightly rhomboidal prism, which is 

 sometimes truncated on all the edges, the truncations in- 

 creasing in breadth till an octagonal prism with nearly 

 equal faces is produced. In other instances the trunca- 

 tions are limited to two edges, in which case an hexaedral 

 prism is the result. This prism is often much flattened, 

 and at times quite thin, the two truncated edgs extend- 

 ing till the original faces are nearly excluded. Some 

 crystals present a very flat tetraedral prism, but as they 

 are much imbedded and difficult to examine it is not 

 easy to ascertain how this variety is produced. The faces 

 of the flat prisms are sometimes rounded, and this con- 

 vexity occasionally extends to the faces of the summits 

 also, so as to produce a very irregular body. In 

 this state it shows the transition into coccolite, that 

 point at which all form vanishes ; the appearance of these 

 curved surfaces resembling that which might be conceived 

 to take place from an incipient solution of a regular crystal ; 

 as if the angles had been rounded off into the neighbour- 

 ing planes. The terminations of the crystals, when 

 perfect, are, in their most simple state, tetraedral pyramids. 



