286 V. K. WKKiHT OliSIDIAN l-'Ho.M K'JOLAXD 



are well develo])e(l. as in speeiinens <S,s4;i() and SS-t;]-^, desci il)c(| nhoxc. 

 In the second case il is important to nolc that the forriiiu- apart of the 

 cavity was a A-erv slow process. 'Hie iirst rujjtnre tool< place wlien the 

 spherulite was small: the rigid walls of the cubical or irregularly shaped 

 cavity thus formed were constantly forced apart, Init continued to g]•()^v 

 as crystallization advanced. The edges of the cube were thin and in eon- 

 tact with the magma, which. hoAve\er, was probably so thick and viscous 

 that less resistance Wfis oifered to the slow forcing apart of the walls of 

 the spherulite than to the formation of gas bubldes adjacent to the spher- 

 ulite. Exam23les of this phenomenon are sliown l)y specimen 88-1-31. It 

 is not possible to determine from the scant evidence at hand the se\eral 

 quantitative factors which are essential to the formation of the type of 

 lithophysal cavities of specimen 88431. 



iM'idence is also presented which shows i-leaily that the d('e])lv etched 

 surfaces on irregular fragments of the obsidian are the residt of ett-hing 

 by hot circulating solutions from which large amounts of hyalite were 

 deposited. Minute crystals of alunite were also deposited during a later 

 stage of circulating solutions. The close resemblance of the surface-etch- 

 ing phenomena thus produced to the surfaces of moldavites and other 

 tcktites is emphasized ; also the mechanics of the etching process by whieh 

 such extraordinary forms are obtained. The distribution of strain within 

 the moldavites is considered briefly. The conclusion is reached that 

 neither the external form of the moldavites nor the distribution of strain 

 within them can he considered to be an indication of their oxtra terrestrial 

 origin, as has been stated by Suess. This conclusion is identical with 

 that recently advanced by ]\rerrill. and the above cxidcncc seivcs to. 

 strenu'then the position taken bv him. 



