10-t L. I ". Pirsson — Artificial Lava-Floio 



or directions through the mass are indicated by parallel chains 

 of little points around which the i>-lass has a minute but differ- 

 ent fracture and wliich tlius interrupt the broad, smooth sur- 

 faces of the usual conchoidal areas. 



The appearance of tliis black glass is so entirely unlike that 

 of the ordinary light-green and transparent bottle glass that 

 formed the flow that it is difficult to believe it is merely a mod- 

 ification of it, though positively so stated. It is, however, as 

 will be presently shown, a curious modification of « light-green 

 transparent bottle glass, though whether of the same melt as 

 tliat which has been previously described as containing the 

 spherulites seems doubtful. The doubt rests on chemical evi- 

 dence. The first glass contains, as mentioned, only a small 

 amount of iron, and a considerable proportion of magnesia, and 

 diopside has crystallized from it; the qualitative analysis of 

 the obsidian indicates much more iron, and while there is 

 abundance of lime, magnesia is present only in minutest traces 

 and as a result woUastonite has crystallized out in a particular 

 layer, as will be described later. Unfortunately, in the lapse 

 of time since the material was formed the possibility of obtain- 

 ing more exact information concerning it and the conditions 

 under which it was produced have been lost. If the statement 

 accompanj'ing the collection is to be trusted, then magmatie 

 differentiation must have occurred, which seems hard to believe. 



Considering the black color, it seemed at first as if the glass 

 must have been of some other melt into which some unusual 

 ingredient had entered which colored it black, or if a part of 

 the flow, then one which had in some way imbibed a coloring 

 constituent. Close inspection of a piece of this obsidian shows, 

 however, that in a place where there are fractures, or cracks, 

 penetrating it, if the mass is so turned to the light that the 

 rays entering it are reflected back from the internal surface of 

 the crack to the e^ye, the black color disappears and the glass, 

 between the crack and the eye, assumes its normal sea-green 

 transparent aspect. 



The black color is due, then, not to a chemically diffused 

 coloring matter, so to speak, in the sense that iron compounds 

 color glass green, manganese purple, or cobalt blue, but is a 

 mechanical effect of some kind, owing to which that light 

 which strikes it and is not immediately reflected from the outer 

 surface, penetrates it and is absorbed. 



Many or most natural obsidians which appear black are 

 found in thin section to be composed of a colorless glass, 

 swarming with specks, or trichites, whose exact nature is 

 unknown, but which many believe to be of magnetite. The 

 idea involved is this: An obsidian is formed because the effu- 

 sion and cooling have been so rapid that the ordinary rock con- 



