THE ORIGIN OP IGNEOUS ROCKS. 
109 
the earth’s superficial shell which the facts of volcanic 
geology demand? 
The hypothesis which he advances to produce such con¬ 
ditions is based on the supposition that the rate of increase 
of pressure within the globe is greater than the rate of in¬ 
crease of temperature, and that the point of fusion is so far 
increased that the substance of the globe is in a state of 
rigidity. Upon a sufficient relief of pressure at any spot the 
fusion point of the material at that place becomes sufficiently 
lowered to allow of its being melted. 
This diminution of pressure, he considers, may be brought 
about by the erosion of the superincumbent material at the 
surface of the earth. “ So that the isolated lakes of fused 
matter which seem to be necessary to fulfill the known geo¬ 
logical conditions may be the direct result of erosion.” 
In considering the “ genesis of volcanic species ” he reviews 
at length Von Waltershausen’s theory of concentric molten 
shells of increasing specific gravity, which theory, he remarks, 
answered the conditions of the natural succession of igneous 
rocks, as far as Von Waltershausen’s idea of that succession 
had progressed; but Von Richthofen, in adopting Von Wal¬ 
tershausen’s theory, was met with the difficulty of account¬ 
ing for the succession of eruptions he had himself established, 
since the highly silicious magmas were erupted between basic 
ones. He was therefore forced to imagine the source of the 
material to have risen from an intermediate position to a 
higher, and then to sink to a still lower one. This, King 
asserts, would have to take place four times during the erup¬ 
tion of the volcanic series since the commencement of Ter¬ 
tiary times; and when the igneous rocks of pre-Cambrian 
and Paleozoic age are considered, which also embrace a 
similar range of chemical variations, not only must the oscil¬ 
lation of the source of eruption have taken place during all 
these ages, but the graduated zone of molten matter must have 
remained unconsolidated for this length of time. Hence, he 
concludes, that “ either this long continued oscillation from 
shell to shell, or, in view of the secular refrigeration, the per- 
