110 J. D. Dana—Resulis of the Earth’s Contraction. 
iron, about as much of lime and also of magnesia, with very 
little potash or soda; and in these constituents there is compara- 
tively little variation. 
Hunt claims that the presence of moisture in many igneous 
rocks sustains the idea of derivation from the fusion of sedi- 
ments, that is, from the fusion of rocks containing water. But 
it has been shown, (on page 107) that the cases of hydrated 
dolerite in the region referred to are local and exceptional, and 
that the ridges are so mixed up with those of the more commo 
anhydrous dolerite, that all must have had one source. His 
and chemical ~~ of limestone and everything else, that 
the product would perhaps be doleritic—a rock low in silica (or 
basic). But these conditions are those of the earth’s liquid en- 
basic or doleritic rocks of Lower Silurian age from the Lake 
* For comparison with the analyses on page 106, I add here others of the 
dolerite (trap) of (I) the Giant’s Causeway, Aa Staffa, and (III) Farée, by Streng 
(Pogg. Ann., xe, 110, 114, 1853), all of which are now supposed to be of Tertiary 
