344 L. V. Pirsson — Geology of Bermuda Island. 



above sea-level, as described in the foregoing paper, and it is 

 evident that the submarine lavas must have had an upward 

 landward extension. The trachytic keratophyre mentioned by 

 Dr. Thomas was not found by the writer ; on the other hand, 

 fragments of a brown glass were observed, in some cases filled 

 with indeterminable microlites. 



Alteration of the Basalts. 



From what has been said in the foregoing account of the 

 petrography of these basaltic lavas it is clear that their situation 

 under sea-water has not preserved them from alteration. We 

 find that water and carbonic acid have been the chief reagents, 

 and carbonates, chlorite, serpentine, titanite, and other sec- 

 ondary minerals are present. Feldspathic minerals, together 

 with melilite and olivine, have been the original ones most 

 attacked and, while biotite, iron ore, and augite in some cases 

 are altered, it is remarkable how often they appear unchanged, 

 especially the augite. The noticeable thing compared with 

 weathering is the lack of oxidation and the accompanying 

 change of ferrous compounds to ferric ones. 



But a certain degree of caution must be observed in ascrib- 

 ing these changes to the action of sea-water alone, for it is 

 conceivable that when a lava flow is blanketed by a later super- 

 incumbent one, the latter while hot and cooling may exert, 

 especially beneath the sea, a hydrothermal action upon the 

 lower older rock. A similar effect might be caused by vapors 

 from the volcanic conduits passing outward through the lavas. 



Thus the alteration observed may result from a combination 

 of causes, and it is difficult to assign to each its proper value. 



Summary. 

 It has been shown in the foregoing that the lavas of Ber- 

 muda, in the area penetrated by the well, consist mainly of 

 meli lite-basalt and types related to the monchiquite group, 

 with a small amount of trachyte. They are, therefore, of 

 alkalic character and consequently correspond with the lavas 

 of many of the great volcanoes that rise from the floors of the 

 abyssal ocean basins, and especially with those of the Atlantic, 

 such as the Azores, Cape Yerde Islands, etc. Bermuda stands 

 in such an isolated position that we are unable here to bring it 

 into relation with any great tectonic lines of the earth, and 

 thus use it for testing those theories which correlate the appear- 

 ance of certain kinds of magmas with definite types of litho- 

 spheric disturbance. The only hint we have in this connection 

 is the elongate shape of the Bermuda mass, which, with its 

 various vents in line, seems to point to their appearance over a 

 fissure, and the eruptions, in most cases, which have taken 

 place from great fissures are of basaltic character. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., June 1914. 



