1892.] Presents. 403 



aspect of the crystallites (where they are seen) does not appear to 

 proceed from a crowding of minute microliths (for a crystallite may 

 sometimes be larger than one of the angite granules), but from its 

 power to depolarise being weak, as if its molecular condition were 

 only slightly anisotropic. It is, therefore, possible that the rock is 

 in the act of passing rather than has actually passed from a glassy 

 condition, though it is not truly a tachylite. 



On comparing the results of artificial processes, described above, 

 with rocks which have solidified from natural fusion, we observe 

 (1) that while quartz and felspar crystals in acid rocks, after having 

 formed, frequently appear to have been partially melted down, the 

 quartzes, as a rule, are not cracked, the inner parts of the felspars are not 

 locally converted into glass. The exterior only as when a soluble sub- 

 stance is acted on by a fluid appears to have been affected ; (2) that, 

 in the case of the basic rocks artificially melted, the glassy part is a 

 true tachylite, but the structure of the devitrified specimen, with its 

 peculiar skeletal crystals of felspar and magnetite and absence of 

 well-defined augite, is unusual in nature.* It is, however, as I know 

 from specimens in my own collection, and as may be seen from 

 Vogelsang's book, rather characteristic of slags and glasses. This 

 appears to be confirmatory of the view commonly entertained that an 

 igneous rock is not liquefied by the action of heat alone, but that 

 water is a contributing agent, and is always present in the magma ; 

 further that the formation of a glass in the process of cooling is facili- 

 tated by the escape of the water, which may explain the comparative 

 rarity of tachylites in nature, and the fact that when they do occur, 

 they are very seldom more than " selvages " to masses of basalt. 



Presents, January 28, 1892. 

 Transactions. 



Baltimore : Johns Hopkins University. Circulars. Vol. XI. 

 No. 94. 4to. Baltimore 1891 ; Studies in Historical and 

 Political Science. Series 10. No. 1. 8vo. Baltimore 1890. 



The University. 



Berne: Naturforschende Gesellschaft. Mittheilungen. 1890. 8vo. 

 Bern 1891. The Society. 



Societe Helvetique des Sciences Naturelles. Nouveaux Memoires. 

 Vol. 30, livr. 2. Vol. 31. 4to. Zurich 1890 ; Actes. Davos 

 1890. Com pte-Rendu, 1889-90. 8vo. Davos 1891 ; Compte- 

 Eendu des Travaux. Davos 1890. 8vo. Geneve 1890. 



The Society. 



* I find a slight approach to it in some vesicular basalts, e.ff., one or two from the 

 Sandwich Islands. See also the figures in Messrs. Judd and Cole's paper on Scotch 

 Basalt-glasR, loc. cit., Plates XIII and XI Y. 



