King’s Systematic Geology of the 40th Parailel. 801 
the pressure on any given point is diminished at a more rapid 
rate than that of the cooling of the couches below, and if this 
removal of pressure proceed far enough, fusion must take 
place; and, being localized, it will form subterranean lakes of 
molten rock. This couche of possible fusion, like all the iso- 
thermal and isobaric couches, must be parallel to the surface 
ing hundreds or thousands of square miles—which have 
formed during single geological epochs. During the Cretaceous 
there accumulated from one to one and one-half miles in thick- 
ness of detrital sediment which probably occupies now a 
larger area than that from which it was eroded, and the follow- 
ing Eocene and Miocene epochs witnessed the enormous out- 
pourings of lava. 
In these subterranean lakes of fused rock, are differentiated 
s. Baron Richthofen, after a 
the varieties of volcanic rock 
Separation, by specific gravity, into a basic, lower couche and 
an acid, ae Lae pe re-solidification by the reéstablish- 
