200 Scientific Proceedings, Royal Dublin Society. 



duriug a very long period, as we have cones of all ages, from very weathered 

 ones to others apparently quite recent.' 



We may begin with a short description of the most recent lava outpouring 

 which I have found in this part of the country. 



The Cerro Diablo is a comparatively small and insignificant cone, only a 

 few hundred feet higher than the surrounding plain ; its summit consists to 

 a large extent of scorias and finely divided cinders ; the latter exist in such 

 quantities as to render the ascent rather difficult. Both scorite and cinders 

 have a very recent appearance, and give one the impression that the volcano 

 was active at no distant date. The lava-fiow from this crater is one of the 

 most extensive in the country ; it is fully fifteen miles in length, in many 

 places upwards of fifty feet thick some miles from the cone, and nearer to 

 the latter much more. It runs in all directions over the plain from the 

 Cerro Diablo, and must have been poured out in a very liquid state, as it 

 is found flowing down the small canadones in minor streams, even nine 

 miles from its point of exit from the vent. The surface is covered by a kind 

 of crust, varying from six inches up to many feet in thickness. Between 

 tins crust and the lava beneath there are empty spaces, extending here and 

 there in all directions ; these spaces are often so large that several people 

 could go down into them and staird erect, with a roof of lava over their 

 heads. 



The lava is deeply fissured everywhere, huge crevasses gaping open, and 

 extending many feet down into the rock. In many eases large portions of 

 the crust have fallen in, revealing deep, gaping holes; in other places, again, 

 the surface is practically unbroken. However, one of the most important points 

 in our present study is that the original cuticle of the lava is absolutely 

 intact and unabraded in the whole of this area ; it has a recent appearance, 

 and shows no sign of patination. There is no other deposit overlying any 

 portion of this lava-sheet, and, with the exception of a few bushes growing 

 here and there in the corners, and some ferns, &c., in the cracks, there is 

 practically no vegetation. After carefully examining this lava-sheet, and 

 walking over a considerable portion of it during several days, I have never 

 found a pebble in any place more than ten yards from its edge. At a place 

 called Pali Aike, situated at the side of the Eio Chico, this lava-sheet pours 

 down the canadones and ouc into the river valley. 



Now, I hope presently to bring evidence to show that the lowest bed of 

 the river valley, with the canadones running into it, represents the last 



' On the characters of the lavas, see Olof Backstrom, Petrographische Beschreibung 

 einiger Basalte von Patagonieu, &c., Bull. Geol. Inst, of Univ. of Upsala, vol. xiii, 

 p. 115. (1915.) 



