chip. in. Obsidian Formation. 63 



has been in contact with a trap-dike, and with a frac- 

 ture of about the same degree of crystalline structure. 

 This rock, as well as the following varieties, easily fuse 

 into a pale glass. The greater part is honey-combed 

 with irregular, angular, cavities, so that the whole has 

 a curious appearance, and some fragments resemble in 

 a remarkable manner silicified logs of decayed wood. 

 This variety, especially where more compact, is often 

 marked with thin whitish streaks, which are either 

 straight or wrap round, one behind the other, the elon- 

 gated carious hollows. 



Secondly, — A bluish gray or pale brown, compact, 

 heavy, homogeneous stone, with an angular, uneven, 

 earthy fracture ; viewed, however, under a lens of high 

 power, the fracture is seen to be distinctly crystalline, 

 and even separate minerals can be distinguished. 



Thirdly, — A stone of the same kind with the last, 

 but streaked with numerous, parallel, slightly tortuous, 

 white lines of the thickness of hairs. These white lines 

 are more crystalline than the parts between them ; and 

 the stone splits along them : they frequently expand 

 into exceedingly thin cavities, which are often only 

 just perceptible with a lens. The matter forming the 

 white lines becomes better crystallised in these cavities, 

 and Prof. Miller was fortunate enough, after several 

 trials, to ascertain that the white crystals, which are the 

 largest, were of quartz, 1 and that the minute green trans- 

 parent needles were augite, or, as they would more 

 generally be called, diopside : besides these crystals, 

 there are some minute, dark specks without a trace of 



The term 'laminated,' in this chapter, is applied in these latter 

 senses ; where a homogeneous rock splits, as in the former sense, in 

 a given direction, like clay-slate, I have used the term ' fissile.' 



1 Professor Miller informs me that the crystals which he measured 

 had the faces P, z, m of the figure (147) given by Haidinger in his 

 Translation of Mohs : and he adds, that it is remarkable, that none of 

 them had the slightest trace of faces r of the regular six-sided prism. 



