F. RUTLET ON THE VITREOUS ROCKS OF MONTANA, U.S.A. 



391 



'30. The Microscopic Characters of the Vitreous Rocks of Montana, 

 U.S.A. By Frank Rutley, Esq., F.G.S. With an Appendix 

 by James Eccles, Esq., F.G.S. (Read April 6, 1881.) 



[Plate XX.] 



The specimens which have supplied the material for this paper were 

 collected by Mr. James Eccles during a tour in Montana. They 

 present so many interesting points of structure that a description of 

 their microscopic characters may prove acceptable to those who are 

 engaged in the study of British vitreous and de vitrified rocks. 

 Among the latter we have already found that certain structural 

 peculiarities may still be clearly recognized, notwithstanding the 

 changes which have led to their more or less complete devitrification. 

 Through these changes, once vitreous rocks assume the character of 

 felstones ; and as it is highly probable that many of our ancient 

 " felstones " and " hornstones" were once vitreous, it becomes im- 

 portant that we should note every structural peculiarity in glassy 

 rocks of recent or late geological age which have undergone little or 

 no change subsequent to their solidification. By doing this we are 

 training our eyes to recognize similar structures in the devitrified 

 obsidians, perlites, and pitchstones which ran over what are pro- 

 bably the earliest land-surfaces of which we have any trace. 

 Zirkel, in his * Microscopic Petrography of the 40th Parallel IS".,' 

 has described a large number of structures met with in the districts 

 surveyed by Messrs. King, Hague, Emmons, and other members of 

 the U.S. Government Survey ; and it is to him we owe much of our 

 knowledge concerning these minute structures. In this country the 

 researches of Prof. Bonney and Mr. Allport have also served to in- 

 crease the interest which rocks of this class will always possess. 

 The more carefully and patiently we study the unaltered examples, 

 the better shall we be able to deal with the questions which concern 

 their older and no longer vitreous representatives ; while researches 

 upon artificial slags or glasses formed under known conditions, and 

 modified by various known causes, either during or subsequent to 

 solidification, will lend additional help in explaining minute struc- 

 tural peculiarities which still remain to be worked out. 



The following is a description of the microscopic characters of 

 ten of the most interesting specimens (which may be regarded as 

 fairly typical of the whole series) collected by Mr. Eccles in this 

 district. In each case the letter M precedes the description of the 

 microscopic characters. 



JS T o. 1. Yellowstone district. Black obsidian. 



A black and perfectly vitreous rock, speckled with rather sparsely 

 disseminated small greyish-white flecks, which are generally imper- 

 fectly developed felspar crystals. 



M. In this section it is transparent and colourless, and is a re- 

 markably homogeneous glass. The porphyritic crvstals (which are 



2d2 • 



