314 D. Forbes — On Volcanos. 



etc., were formed prior to the final disappearance of tlie ice, is 



Fig. 3. Flat-bottomed lugs cut into the north side of the Muilrea hills, 

 proved by the rocks forming them being dressed, polished, grooved, 

 and etched.^ 



In the Silurian and granite countries the action of the sea is not 

 so apparent, and a casual observer might not remark any traces of it ; 

 however, on the hill-side in many places are sloping terraces, which 

 on comparison are found to be on similar levels to the terraces, etc., 

 just now mentioned, and consequently having a like origin, but now 

 modified or nearly obliterated by the subsequent meteoric abrasion. 



It has now been demonstrated that the features of Yar-connaught 

 as they now exist are due to the joint work of the three denudants, 

 namely : marine action, ice, and meteoric abrasion ; yet in parts of that 

 country there are now scarcely any traces of marine action, although 

 this force must have acted in those portions prior to the ice and the 

 subsequent meteoric abrasion, as much, if not more, than in the 

 tracts where the traces are still conspicuous. Part of this country 

 in which the traces of marine denudation have been nearly oblite- 

 rated is formed of Silurian rocks, somewhat similar in character 

 to the grit slabs and slates of Devon (Devonian rocks), and in this 

 part of Yar-connaught the valleys are of the same character, and are 

 very similar to those of Devon. Is it, therefore, unreasonable to 

 suggest that the three forces which jointly formed the present fea- 

 tures of Yar-connaught, were also combined in carving out the pre- 

 sent surface of Devon, although all conspicuous traces of ice and 

 marine action may have been since obliterated by meteoric abrasion.^ 



IV. — On Volcanos.'' 

 By David Forbes, F.R.S., etc. 



HAVING- listened with much pleasure to several able lectures on 

 Geology and Palaeontology which have already been delivered 

 in this Hall, I believe I am correct in affirming, that the general 

 tendency of these discourses has been altogether in one direction ; 



1 In Ireland also there are at least two or three sets of terraces that were formed 

 subsequent to the disappearance of the mass of the ice, but probably prior to the 

 ^nal disappearance of the snow on the uplands and the ice-streams in the hill 

 valleys. (See Memoir Geol. Survey, Ireland, ex. sheet, 105.) 



2 There are sloping terraces on some of the hill-sides of Devon and Cornwall, but 

 whether they occur in systems, and consequently are due to marine action, has still to 

 be worked out. 



3 Being the substance of a lecture delivered in St. George's Hall, June 19, 1870. 



