S50 Sir Geov^e Mackenzie's Observations on Menp'e's 
I believe, that all those streams of lava, as they have been called, 
winch appear in tlie form of beds, must have been formed un- 
der peculiar circumstances, which were calculated to prevent the 
rugged appearance which the surface lava commonly assumes, 
^and to induce that of regular beds. It might be supposed, 
tliat the rugged surface of lava has been caused by its descending 
along an inclined plane. But much of that which Menge says he 
could not behold without horror, has proceeded over a level 
country ; and I saw many other lavas of the same aspect, in si- 
milar circumstances. To explain the different appearances, I 
suggested the idea of submarine lavas ; and I am still inclined 
to adhere to it, and to that of subterraneous lavas also, as best 
calculated to explain the various appearances of trap-rocks, and 
their relation to lava. 
k I still doubt, that any lava that has flowed in the open air, has 
assumed the columnar form All that I saw tending to that form, 
in the Guldbringe Syssel, certainly had not flowed on the sur- 
face in the open air. I have, indeed, seen partial, but feeble, 
tendencies in detached masses to this form, but nothing at all re- 
sembling the regular appearance of columnar beds, such as are 
to be seen at Stappen, and in other parts of the Snasfpll Syssel, 
as well as in other parts of Iceland. 
It is highly satisfactory to me to have heard Menge give the 
same opinion that I had formed, with respect to certain rocks 
appearing to have been melted in situ^ and this without his hav- 
ing seen my Observations on the Mineralogy of Iceland. What 
he calls a Felspar-lava, is not Greenstone, as you suppose, by 
placing that name in a parenthesis. It might, however, have 
been a greenstone previously to the action of heat upon' it. 
I confess tliat I am not able fully to comprehend what 
Menge intended to convey by aqueous volcanoes ; and connect- 
ing, as he does in his letter to you, pseudo-volcanic rocks with 
water, is contrary to the idea wn have of pseudo-volcanoes. I 
suspect that when he speaks of having found in his journey 
northward, a perpetual alternation of true volcanic and pseudo- 
volcanic rocks, he means a succession as he advanced, not that the 
one was superimposed on the other. When I first saw the ironshot 
trap-tuff, I also took it for sandstone, as many others have on 
looking at the specimens I brought to this country from Ice- 
land and Faroe. I suppose you do not consider the application 
