Account of his Journey in Iceland, 
of the word Neptunian, proper in describing debris brought 
down by glaciers. Neptunian, I have always understood to be 
confined to formations from solution, in contradistinction to 
Plutonian. When Menge says, All the members of the 
trap series are arranged (at Skagafiord) above each other in 
horizontal beds,"^ and some may be inclined to view these 
beds as streams of lava, formed at different periods ;” he seems 
inclined to be of the opinion I have offered in my account of 
Iceland. He appears to consider only one mode of formation 
possible for lava, and not to take into account the effects of 
compression, or those of submarine or subterraheous flow- 
ing. In conversation Menge informed me, that he was per- 
petually puzzled what to think of Iceland; that he i^was 
one day tempted to consider the whole as of igneous forma- 
tion, as I do; and on another, that it was partly of aqueous 
formation. He is to return to this singular country ; and it is 
very probable that he will at last determine, that it will be bet- 
ter to ascribe the whole formation to one cause. Heat, acting under 
a variety of circumstances, and occasionally combined with wa- 
ter, than to insist on the exclusive operation of water in one spot, 
and of heat in another, thus rendering Iceland a vast collection, 
of anomalies in geology. I will never cease pressing you to visit 
Iceland, because I am convinced that no' geologist can form 
adequate conceptions of the agency of heat, without having 
seen this, or some country similar to it. 
There is no doubt whatever, that some of the products of the 
hot-springs are in appearance glassy ; and I should not wonder 
if Obsidian, properly so called were proved to have had its 
origin in water extremely hot. 
I do not know what is meant by volcanic water. There is a 
spot in the Guldbringe Syssel, in the midst of what Menge 
aptly names the Congealed Pandemonium, where a great quan- 
tity of water rises, and forming a considerable stream, flows a 
mile or two, and is then lost among the lava, which he did not 
see. But whether this be what Menge calls volcanic water, I 
* You are aware, that I pointed out a marked distinction between two sub- 
stances hitherto indiscriminately called Obsidian, and one of which I propose to 
name Pumice-glass, from pumice being formed by melting it, and that substance 
being alway.s found connected with it. They are easily distinguished by the eye. 
