AND THE WEST INDIAN ERUPTIONS OF 1902. 



223 



rocks co(jler than lava at their ])ase and becoming continuously 

 cooler to the surface, even with a conduit throuohout, is much too 

 great a tliickness for lava to pass through unsoliditied. A lava 

 column of 100 feet diameter would give a much greater flow 

 than the great majority of lava emissions either in the present 

 epoch or in past geological periods. Yet this would be a mere 

 thread in proportion to its length of 30 miles. From the records 

 of many eruptions, I find that an ascending movement of 1 foot 

 per second will give an unusually rapid How of the more mobile 

 or basic lava, yet at this rapid rate the lava would require 44 

 hours to travel from a base of 30 miles depth, all the time in 

 contact with cooler and, as it rose, with increasingly cooler rocks. 

 Fluid lava has little excess of heat over the fusion point and 

 consequently with little loss of heat it solidifies. Such a 

 colunni of lava would therefore solidify long before reaching the 

 surface. But the great majority of lava-flows are much 

 smaller than one from 100 feet column, and in a great number 

 of cases are very small flows. These must, therefore, be from 

 very thin columns or from very slowly ascending columns, and 

 in either case the possibility of the fluid lava reaching the 

 surface must 1)e dependent on a comparatively small depth of 

 source. 



There is, again, another objection that seems to me to be even 

 more conclusive against a 30 miles deptli of the source of lava. 

 This is that there could not possibly be a fissure or conduit 

 through 30 miles of rocks, or, indeed, through rocks at all 

 approaching that thickness. The weight of a column of ordinary 

 rock of 1 square foot section is 400 tons per mile, or 800 tons 

 for two miles. This exceeds the crushing weight of granite, 

 which is 720 tons per cubic foot. Although in great mass, from 

 lateral resistance of the contiguous rock-masses giving a 

 counteracting resultant force, the full weight of 800 tons would 

 not be exerted at 2 miles depth, yet it would at a somewhat 

 greater depth, and thus, as M. Tresca has shown, at a depth of 

 more than a few miles from the surface, the rocks, although 

 solid, will "flow," or move horizontally, if laterally unsupported, 

 and consequently cracks or fissures at these depths are impossible. 

 Hence it will be quite safe to say that no openings exists below 

 a few miles from the surface. 



Thus it would appear that both the central source of lava and 

 the 30 miles distant source of lava, must be given up, and with 

 them, of course, all hypothesis founded on those bases. This 

 narrows and simplifies the inquiry very greatly, since, with the 

 elimination of these hypotheses, we can no longer regard the 



