RUM. MINERALS. 505 



case the transparent variety puts on the external aspect 

 with the lustre of jet ; while the opake one preserves its 

 dull surface and more nearly resembles black chalk. 

 Notwithstanding this change the mineral when in small 

 fragments still continues to transmit light. The first 

 variety remains perfectly translucent and presents in some 

 specimens the fine brown orange of cinnamon stone, in 

 others a rich bottle or olive green. The other appears 

 also of an olive green, but is not more translucent than 

 wax of the same thickness. When powdered, the one 

 is of a snuffy brown; the other, of a dirty olive. The 

 fracture of the first variety is generally conchoidal, that 

 of the second is commonly intermediate between the 

 conchoidal and granular. It is so soft as to be scratched 

 by a quill, and is brittle ; easily breaking into minute 

 irregular fragments. The specific gravity is 2,020. 



With respect to its chemical habits it remains unchanged 

 before the blowpipe ; neither cracking nor sensibly alter- 

 ing its colour or translucency. It is apparently as refrac- 

 tory as quartz ; a remarkable circumstance, when the 

 quantity of iron in it is considered. It is acted on by 

 muriatic acid, giving indications of a considerable pro- 

 portion of iron, with a little alumina ; but the principal 

 constituent appears to be silica. There are no traces 

 of lime or of manganese. The very minute quantity I pos- 

 sessed for examination prevents any more accurate detail 

 of its composition. 



It is found imbedded in the amygdaloids of the 

 cliffs of Scuir more, the base being either a basalt, or a 

 black indurated claystone. The nodules are generally 

 round and vary from the size of a radish seed to that of 

 a pea or upwards. Occasionally they are oblong and 

 compressed, and sometimes scale off in concentric crusts. 

 In a few instances they are hollow within, the inte- 

 rior surface having a blistered aspect ; or else the cavity 

 of the amygdaloid is covered with the substance in a 

 form resembling that of an exudation. More rarely 



