166 Mr Menge’s Mineraloglcal Journey in Iceland. 
Neptunian formation, which appears to be produced by hot- 
springs. In the whole volcanic region from the Guldbringe 
Syssel to Myvatn the formation of the hot-springs is abundant. 
We often observe members of the neptunian traps alternate with 
those of the volcanic-neptunian series, and also beds of true 
lava alternating with volcanic-neptunian trap. The first occurs 
at Essian, Mossfell, Blaufell, Ericks and Bald-Yokull, Tinda- 
stoll, &c. ; the latter at Eyafialla-Jokull, Kiolfell, Hoffsjokull, 
and in nearly all the hills of Myvatn. In the whole southern 
track examined by Sir George Mackenzie, there is no pure 
neptunian trap, with the exception of some members of the se^ 
ries at Borgarfiord, which alternate with volcanic-neptunian 
rocks. 
The lakes of Thingvalla-vatn, Huitaarvatn, Fiskevatn, Liso- 
vatn, Myvatn, &c. were formerly districts abounding in hot- 
springs, which have fallen in, and formed the hollows now occu- 
pied by the water of the lakes. Beside the well-known hot-spring 
districts of Reikianess, Krysuvick, Geyser, Reikum, &c. in the 
south, there are many others in the middle of the island, and a 
great many in the neighbourhood of Myvatn, of which the most 
considerable are Hueravellir, Naumafell, Oxehver, Testareiker, 
and those at Odadarhaun. 
W e cannot maintain, that the silica in these hot-springs is de- 
rived from a quartzose solution, because quartz is one of the 
rarest minerals in Iceland. The siliceous-sinter, with all the 
other substances afforded by the siliceous water, viz. sulphur, 
gypsum, alum, bole, &c. disappear as soon as the water cools, 
and the residuum is trap-porphyry, basalt, and even amygda- 
loid. 
The neptunian formation predominates in east and west Ice- 
land, yet not without some occasional intermixture of volcanic- 
neptunian rocks. The greater number of veins that occur in 
these rocks, are volcanic-neptunian. 
Observations by the Editor. 
From the preceding short account, it appears, that Iceland 
is formed of four classes of rocks, viz. Neptunian or common 
Trap rocks, Volcanic-neptunian rocks, true Volcanic rocks, and 
Alluvial rocks. 
