OCR Text |
Show THE EXPLANATION OP THE SEQUENCE. 131 gross weight of this overlying cover of solid rocks, I conceive, which presses the lava upward through any passage where it can find vent. It will follow, then, as a corollary, that the lava will rise to the surface or not according to its density. If it be lighter than the mean density of the rock above its reservoir, it will reach the surface and nothing can keep it in; if it be heavier than the overlying rock, it will never reach the surface. IV. We come now to the explanation of the sequence of volcanic rocks. In order that any eruption of lava may take place two preliminary conditions are requisite: First. The rocks must be fused. Second. The density of the lavas must be less than that of the overlying rocks. Having shown from independent considerations that the proximate cause of volcanic activity may be a local rise of temperature in the deeply-seated rocks, it only remains to follow the obvious phases of the process. We know that the volcanic rocks vary within tolerably ample limits as to their chemical constitution, and that associated with these chemical differences are notable differences of physical properties. Some are more fusible than others and some are heavier than others. We also presume that prior to eruption these different rocks were within the earth separated as if in strata or in macula. Imagining, then, a rise of temperature in a nether region where the constitution of the magma is variableâ€"here very siliceous, there very basic, with many intermediate varieties, all arranged in any arbitrary manner and in each other's neighborhoodâ€"it is quite certain that not all of these magmas would be both fused and sufficiently expanded by heat to be ready for eruption at the same time. The more refractory rocks might not be melted or the heavier ones might not be sufficiently expanded. There would, therefore, be some selection as to the order in which they would become eruptible. But upon what principle would the selection be made ? The acid rocks are known to have the highest melting temperature, but the basic rocks in the cold state have the highest specific gravity. It is just possible that the acid rocks may be light enough to erupt at an early stage of the process but are not yet melted, and that the basic rocks may be melted but must await a further expansion in order to reach the surface. The first selection would then fall upon some intermediate rock. Let us |