The hypothesis suggested by Mr. Scrope
may, perhaps, afford a solution of this problem. The minerals, he observes,
which abound in basalt are of greater specific gravity than those composing the
feldspathic lavas; thus, for example, hornblende, augite, and olivine are each
more than three times the weight of water; whereas common feldspar and albite
have each scarcely more than 2 1/2 times the specific gravity of water; and the
difference is increased in consequence of there being much more iron in a
metallic state in basalt and greenstone than in trachyte and other allied
feldspathic lavas. If, therefore, a large quantity of rock be melted up in the
bowels of the earth by volcanic heat, the denser ingredients of the boiling
fluid may sink to the bottom, and the lighter remaining above would in that case
be first propelled upward to the surface by the expansive power of gases. Those
materials, therefore, which occupy the lowest place in the subterranean
reservoir will always be emitted last, and take the uppermost place on the
exterior of the earth's crust.
TEST BY INCLUDED FRAGMENTS.
Pages:
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973