204 L. V. Pirsson — Geology of Bermuda Island ; 



into consideration, it seems most probable that the mass as a 

 whole is composed chiefly of basaltic lavas. 



If we assume that the main platform represents the trun- 

 cated top of a single volcanic peak and continue the proh'le 

 upward by the dotted lines shown in figure 2, then this peak 

 would have risen 11,500 feet above the present sea-level, or 

 about 26,000 feet above its base on the sea floor. This com- 

 pares with Hawaii, whose peaks reach a height of 30,000 feet 

 above the ocean bottom, and Samoa, 25,000 feet. It is quite 

 as probable, however, that this platform was cut through 

 several coalescing volcanic cones and in this case the height 

 would not have been so great. 



The definite alignment of the Bermuda volcanoes is similar 

 to that observed in numbers of volcanic island groups in the 

 Pacific and suggests their situation upon a great fissure or 

 fault-line in the lithosphere. If the whole Bermuda mass is 

 composed entirely of volcanic material, as seems probable 

 from its isolated position, it represents an extrusion of at 

 least, roughly, 2,500 cubic miles of igneous magma, and it is, 

 therefore, one of the larger volcanic occurrences of the world. 



The debris of the land that has been cut away by atmospheric 

 erosion and the work of the sea must have been in consider- 

 able part distributed over the flanks of the volcano. The 

 coarsest particles of it would lie nearest to the shore and thus 

 highest up. This is indicated by the character of the material 

 found in the boring, which seems to have been largely gravel 

 and coarse sand, judging by the size of the grains and pebbles. 

 This of course would tend to broaden somewhat the submarine 

 platform and increase its size, but it has not been a very con- 

 siderable factor in this direction, for the depth of the whole 

 deposit is only about 300 feet and the point where the boring 

 was made is one where it might reasonably be assumed to be 

 as thick as anywhere over the platform. Compared with the 

 bulk of the mass such a deposit would be a mere skin over its 

 truncated top and flanks. This assumption, if correct, would 

 tend to suggest that the former land was composed rather of 

 several lower, smaller cones than the huge one indicated in 

 the profile. 



The present depth of water over the different banks and the 

 extent to which the bore penetrates before passing through the 

 limestones into water-worn debris seems to indicate a change 

 in water level of perhaps 200 feet since the time when the 

 waves were attacking the coast line of the former island. 

 This point will be considered more fully by Doctor Vaughan. 



It has been considered, in drawing the section, that where 

 amygdaloidal lava was encountered, this would indicate the 

 upper surface of a flow. According to this, seven flows are 



