602 PROCEEDINGS OF BOSTON MEETING. 



That the rock of the Volcano projectiles does not belong in the trachyte series- 

 but is. chemically like the dacites, is clear from the above analyses. Analyses II 

 and III are of typical dacites, with which this rock is chemically almost identical. 

 The domite of the Auvergne (IV) and the Monte Amiata trachyte (V) contain 

 much more potash. The pantellerites contain more potash and iron and much 

 less alumina and lime. The other trachyte types show chemically even less 

 affinity with the rock under consideration. 



Although the rock under consideration is chemically identical with the dacites, 

 it lacks tlie characteristic quartz phenocrysts of that type. Further, the study of 

 the sections shows that this rock belongs, mineralogically, in the trachyte series 

 with the pantellerites. Anorthoclase is the predominating felspar and mono- 

 clinic pyroxene, which is the rarest non-felspathic essential constituent of the 

 dacites, is here beside felspar the only essential constituent of the rock. This rock 

 does not therefore correspond with any of the established rock types. Miner- 

 alogically it is a trachyte, while chemically it is a dacite. I propose for this new 

 type the name Volcanite after the crater from which it has been ejected— Vol, 

 cano — the most symmetrical and beautiful of ah cinder cones. 



Professor J, P. Iddings has demonstrated that one and the same rock magma 

 may under different conditions consolidate in different mineral aggregates. - The 

 rock type which arises in a given case depends on the manner of geological occur- 

 rence and on the physical conditions attending the consolidation. These latter 

 may be the pressure, the temperature, the rate of radiation, the mineralizers pres- 

 ent, etcetera. In the type before us the development of anorthoclase and mono- 

 clinic pyroxene, and not quartz, acid plagioclase, and biotite or hornblende, as 

 is usual with magmas of this chemical composition, may in part be due to the 

 peculiar manner of formation of the " bread-crust" projectiles. 



The following paper was read by title : 



FURTHER NOTES ON THE OCCURRENCE OF ALBERTITE IN NEW BRUNSWICK, 



CANADA 



I 



BY H. P. H. BRUMELL 



The Society took a recess for luncheon, which was served in the build- 

 ing by the resident Fellows, and reconvened in the Geological Lecture- 

 room at 1.30 p m. The first paper presented at the afternoon session 

 was — 



ALTERATIONS OF SILICATES IN GNEISS A T WORCESTER, MASSACHUSETTS 



BY JIOMER T. FULLER 



* J. P. Iddings : The mineral composition and geological 0('cm-renee of certain igneous rocks. 

 Bull. Phil. Soc. Washington, 1889, vol. xi, pp. 191-220. Also Iddings: On the crystallization of 

 igneous rocks, ibid., p. 90; and. The Eruptive Rocks of Electric Peak and Sepulchre Mountain, 

 Yellowstone National Park, 12th Ann. Rep. Director U. S. Geol. Survey, 1892, pp. 569-CG4. 



