212 TV. G. Locke — Volcanic History of Iceland. 



they are to be attributed can be neither a great Scandinavian Ice- 

 sheet, nor local glaciers. As Dr. Croll observes (" Climate and Time," 

 p. 451), when referring to Shetland glaciation, the islands have no 

 hills or valleys to form a gathering ground for glaciers; and even if 

 in these islands glaciers could have been formed, they never could 

 have crossed seas, with a depth of 50 or 60 fathoms, and for distances 

 of from 12 to 20 miles, carrying Boulders and drift. The circum- 

 stances of the case seem rather to point to a time, when the islands 

 were submerged in a sea with floating-ice, which, by the action of 

 tides and currents, rafted fragments of rocks and rubbish from 

 one island to another. 



I very much regret if, in this my reply, I have said things which 

 are disagreeable to the authors of this paper, especially considering 

 the praiseworthy labour they bestowed, during four successive years, 

 in examining the geology of these islands. Had the questions raised 

 been merely as to the proper nomenclature of certain rocks, or of the 

 fossils in them, there would not have been the same necessity for 

 criticism. But the theory they have advocated regarding glaciation 

 by this Scandinavian Ice-sheet, must, if true, entail important 

 geological conclusions. The authors of the paper speak of the 

 " merit " of Dr. Croll in having suggested this theory ; and they 

 probably went to the Shetlands so strongly prepossessed in favour 

 of it, as to feel it to be alike a duty and a pleasure to gather facts 

 tending to support the theory, and to supply arguments, and 

 suggest ingenious hypotheses, with the view of obviating objections. 

 They merely adopted Dr. Croll's theory, and he is more responsible 

 for it than they are. 



YI. — V0LCA.NIC History of Iceland. 

 By "Wm. George Lock. 



LOOKING carefully through Herra Thoroddsen's researchful and 

 excellent paper, with the above heading, published in the 

 October (1880) Number of the Geological Magazine, I notice 

 that two or three errors have crept in. 



The most important oversight is Danish measure being given as 



English,thus the area of the large central lava-desert, the Oda/^ahraun, 

 is given at " sixty geographical square miles," whereas its real area 

 is at least 1400 English square miles ; while the area of Askja, the 

 crater of a large volcano in the centre of the lava-desert, is given 

 at " one square geographical mile," whereas it is at least twenty- 

 three English square miles. 



Having visited AsJy'a twice, the first time in 1878, and again last 

 summer, I should like to say a word or two about it, as nothing 

 like an intelligible description of the volcano has, as yet, been 

 published in English ; and scientific men, therefore, have no idea of 

 the magnitude and exact position of one which, as recently as 1875, 

 erupted such a vast quantity of pumice and ash, that over 2000 

 square miles of country were covered to a depth varying from 

 several feet to two inches ; a lava-flood bursting forth, presumably 



