chap. in. Obsidian Formation. 67 



The sphaerulites are either white and translucent, or 

 dark brown and opaque ; the former are quite spherical, 

 of small size, and distinctly radiated from their centre. 

 The dark brown sphaerulites are less perfectly round, 

 and vary in diameter from the -20-th to ^-th of an inch ; 

 when broken they exhibit towards their centres, which 

 are whitish, an obscure radiating structure ; two of 

 them when united, sometimes have only one central 

 point of radiation ; there is occasionally a trace of a 

 hollow or crevice in their centres. They stand either 

 separately, or are united two or three or many together 

 into irregular groups, or more commonly into layers, 

 parallel to the stratification of the mass. This union 

 in many cases is so perfect, that the two sides of the 

 layer thus formed, are quite even ; and these layers, as 

 they become less brown and opaque, cannot be distin- 

 guished from the alternating layers of the pale-coloured 

 feldspathic stone. The sphasrulites, when not united, 

 are generally compressed in the plane of the lamination 

 of the mass ; and in this same plane, they are often 

 marked internally, by zones of different shades of 



No. 6. 



Opaque brown sphaerulites, drawn on an enlarged scale. The upper ones are ex- 

 ternally marked with parallel ridges. The internal radiating structure of the 

 lower ones, is much too plainly represented. 



colour, and externally by small ridges and furrows. In 

 the upper part of the accompanying woodcut, the 



