212 AUSTIN F. ROGERS 



northwest corner of San Bernardino County, to the northwest of 

 Barstow, probably within 40 to 50 miles from Barstow." The 

 geological map of California compiled by Professor James Perrin 

 Smith and published by the Cahfornia State Mining Bureau shows 

 a number of outcrops of volcanic rocks northwest of Barstow, 

 and so Mr. Wilke is probably correct as to the approximate locality. 



THE OBSIDIAN AND ITS LITHOPHYSAE 



The specimens consist of dark gray to almost black obsidian 

 (w = 1.483 ='=.003) with a faint banded structure and bright vitreous 

 luster. In thin fragments the obsidian shows on microscopic 

 examination numerous, minute, rod-shaped crystallites arranged 

 in parallel lines and a few microlites. 



The obsidian contains many light gray spherulites which range 

 from 5 to 25 mm. in diameter and are fairly evenly distributed, 

 though sometimes two of them coalesce. The spherulites are more 

 or less flattened; some of them are elongate, and most of them are 

 somewhat irregular in outhne. With a few exceptions they are 

 hollow and hence may be called lithophysae. It is estimated that 

 the spherulitic material occupies only about a fifth to a half of the 

 original space. The spherulites are for the most part ruptured, 

 and deep cracks extend almost to their margin, so that the spaces 

 left are usually very irregular in outline. The cracks give some of 

 the lithophysae a superficial resemblance to mud-cracks. The 

 thickness of the spherulitic material is often only about a tenth of 

 the radius of the original cavity. 



^accompanying the obsidian specimens were a number of loose 

 spherulites or lithophysae, which had been broken out of the 

 obsidian. They are much more regular than the imbedded litho- 

 physae just described. One of these is shown in the photographic 

 enlargement (Fig. i). They have a more or less well-defined 

 hollow space at the center and show a sHght tendency to concentric 

 structure, as may be noticed in the photograph. 



It seems probable that both the expansive force of the gas 

 within the cavity and the tension due to coohng of the surrounding 

 glass were responsible for the formation of the lithophysae.^ 



^ For a discussion of the origin of lithophysae, see paper by Wright, Bull. Geol. 

 Soc. Am., Vol. XXVI (1915), pp. 255-86. 



