148 Scientific Proceedings, Royal Dublin Society. 



The annexed diagram is a section of the Puy de Dome, 

 constructed in accordance with this idea, and it certainly appears 

 to me that the hypothesis is strengthened by the fact, that the 

 mountain rests upon a base of ashes and scoriae through which 

 the road winds before one commences to make the ascent of the 

 steepest part of the hill, which commences very abruptly. 



Section through Puy de Dome. 



;< X ^^ 



a. Cap of Domite. 



b. Cone of ashes and scori© 



c. Much older basalt from Mont Dore district. 



d. Granite forming fundamental plateau. 



It is quite easy on simple hydrostatical principles, and with 

 the aid of the data which I have given, to verify at any rate the 

 'possibility of this explanation, and thus to entitle it to hold its 

 own until a more feasible one is proposed, or this shall have been 

 put to the test of experiment. It is a mere question of the differ- 

 ence in hydrostatic pressure between a column of molten lava of 

 a specific gravity of S'O, and that of one of the same height, but 

 having a densityof only 2-5. ISrow,in one of these cones the bursting 

 pressure was attained when the heavier lava rose to a height of 

 800 feet in the crater, and was represented by a pressure at the 

 base of the column of 80 atmospheres or 1,200 lbs. on the square 

 inch. (I presume these cones gave way somewhere near the base, 



