146 Scientific Proceedings, Royal Dublin Society. 



Domite ; this must at any rate be left to mineralogists, and 

 even if I were capable of discussing the subject, I should be 

 sorry in any way to deprive them of such an interesting bone of 

 contention ; but all I shall attempt to do is to advance what I 

 hope will be considered a reasonable theory of the internal 

 structure and mode of origin of these strange elevations. 



Just to mention the older theories — Desmarest considered them 

 to have been originally composed of granite, which was after- 

 wards calcined in situ by a volcanic conflagration environing it ! 

 This hypothesis requiring, I conceive, for its elaboration the vivid 

 but superficial imagination of a Frenchman. Others, Von Buch 

 and Humboldt included, thought that they were enormous 

 bubbles blown up by subterranean gases and solidifying in that 

 shape — a theory one would suppose considerably more hollow 

 and unstable than it would fain have made the mountains them- 

 selves. 



Scrope draws attention to the probable viscosity of the mass 

 of Domite when in a molten state, in contradistinction to the 

 comparative fluidity of basaltic lavas; but does not explain, I 

 think, how this viscosity was able to raise mountains some 2,000 

 or 8,000 feet above the level of the surrounding plain. I proceed 

 to lay before you the following considerations on the subject. 



This Trachyte or Domite being composed almost exclusively 

 of felspar, with crystals of augite and mica in small proportion, 

 has a comparatively low specific gravity, which is again much 

 lessened by its very granular and porous character and the con- 

 tained bubbles of air, so that its specific gravity is as low as 2-50, 

 or even lower ; but this will be sufficient for my purpose.* 

 When we come to examine the basalt of which the more recent 

 lava-flows are composed, we find its specific gravity to be much 

 greater, namely — 3'0 to 3"10. I will take the average figure or 30. 



* Since writing the above I have taken the actual density of a piece of Domite, 

 and its specific gravity is exactlj' 2*5. This, however, is after it had been thoroughly 

 soaked in water, and all the air which lodges in its pores expelled. How large a 

 quantity of air is contained in this rock may be judged from the fact that on putting 

 this piece into water it appeared to effervesce, the air escaping in such abundance. 

 Scrope states that when a shower of rain falls on these Domite hills a loud hissing 

 noise is produced, which I can well believe, although we were not so fortunate as 

 to witness the phenomenon. This all points to its density while in the molten state, 

 having been very much less than this; but, as I said, this actual density is sufficient 

 for my purpose. 



