Vol. 57.] MOUNT GIPvNAR, JUNAGARH. 39 



same district, in. which an undecomposed isotropic matrix still exists 

 [11] p. 395; and Kemp & Marsters have described similar rocks 

 from the Lake Champlain region [13]. 



Iq 1896 Mr. L. V. Pirsson, of the United States Geological Survey, 

 advanced the contention that the supposed glassy matrix of the 

 monchiquites consisted in fact of analcime. He had been investi- 

 gating rocks of this class in Montana, and at first accepted the 

 view that they consisted of ferromagnesian silicates in a glassv base 

 [19]; but 



* when the rocks were studied in connection with their geological mode of 

 occurrence, it became a source of perplexity as to why such basic magmas . . . 

 should have formed so much glass,' 



while the more acid types which accompany them do not, under 

 similar conditions, present glassy forms. 



Optical methods failed to yield any decisive results as to the real 

 nature of the supposed glass, and reference was made to the 

 chemical composition of the rock as shown b}^ the analyses. After 

 allowing for the chemical constituents of the minerals known to be 

 present, the remaining sihca and the alkalies, alumina, and water 

 were found to correspond with the formula of analcime as nearly 

 as could be expected, taking into account the fact that the com- 

 position of the ferromagnesian silicates was not exactly known. 



The analj'sis of the isotropic matrix in the monchiquites described 

 by Hunter & Rosenbusch was dealt with in a similar manner. 

 The iron, lime, and magnesia, as well as the silica needed to form 

 bisilicates with them, were removed ; and the proportions of the 

 remainder of the silica and of the alumina, alkali, and water were 

 found to be those of analcime : — 



* It has the exact chemical composition, the exact specific gravity, '^ the 

 property of gelatinizing with acids, and the optical properties of analcite ; and 

 must therefore be that mineral, and not a pitehstone-glass, as had formerly 

 bt-en supposed ' [19] pp. 682-83. 



Pirsson further states that in the original Brazilian monchiquite 



' the analcite often shows a tendency to crystal-form by the production of 

 areas which are free from the larger prisms of the ferromagnesian minerals, 

 the latter being arranged around them in wreaths. The areas thus resemble 

 phenocrvsts of leueite, and they are in reality phenocrjsts of analcite. They 

 are sprinkled full of the microlites of hornblende described by Rosenbusch, 

 which do not, however, show any tendency to the zonal arrangement shown by 

 such inclusions in leueite.' 



He considers the analcime to be primary, because of the fresh 

 unaltered character of the minerals (op. cit. p. 686), and has difficulty 

 in understanding how the base could have undergone a thorough 

 chemical change and decomposition without the minerals being 

 affected in the slightest degree, especially the olivine. 



^ The specific gravity of analcime varies between 2' 15 and 2*28, which is 

 less than that. (2-31) of the base of the type-inonchiqnite ; but the latter 

 contains microlites of lieavier minerals. It is, however, not necessary that a 

 glass of the same coDiposiiion as analcime should have a different specific 

 gravity. 



