THE EXPLANATION OP THE SEQUENCE. 131 



gross weight of this overlying cover of solid rock8, 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 sur- 

 face 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 vol- 

 canic 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 chem- 

 ical 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 macules. Imagining, then, a rise of temperature in a nether region 

 where the constitution of the magma is variable — here very siliceous, there 

 very basic, with many intermediate varieties, all arranged in any arbitrary 

 manner and in each other's neighborhood — 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 I 

 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 



