474 PETROLOGICAL ABSTRACTS AND REVIEWS 
a given amount of vesiculation, are adduced, and also for the velocity of 
solitary bubbles rising through a magma of given viscosity and at a given 
pressure; while the rate of rise of a mass of vesiculated lava in a non- 
vesiculate magma is estimated by the analogy of solid spheres moving 
under gravity in viscous liquids. These computations indicate that such 
a process is eminently possible, and it is regarded by the author as 
essential to the prolonged maintenance of activity. The action of the 
gases is further extended in the conception of the volcanic furnace. If 
the juvenile gases accumulate at the top of the magmatic chamber as 
indicated they will become concentrated in the actual volcanic pipe, and, 
according to the law of mass action, exothermic reactions on a large 
scale between the gases themselves are to be expected. Moreover, there 
may be other, and endothermic, compounds formed in the primitive 
earth, and the energy thus potentialized in the substratum will be liber- 
ated at these levels of greatly lessened pressure. Even before the magma 
reaches the surface such conditions will exist in the cupolas of the roof, 
and these highly heated gases, still under great pressure, will exercise a 
tremendous blowpiping action. ‘The rdéle of the gases in volcanic action 
is therefore a very important one. 
The gas-fluxing hypothesis also explains the small size of observed 
volcanic pipes, since the slow passage of relatively small amounts of gas 
would not be expected to open a large vent. A very minute deformation 
would have a great effect on the rise of the gas, which would set all of the 
aforementioned processes in operation and reopen a dormant vent, so that 
the difficulty of explaining periodic vulcanism is lessened. Magmatic 
differentiation in central vents is to be largely explained upon principles 
which have been demonstrated in plutonic geology. Lava is however 
especially prone to differentiation, owing to the fact that it is kept fluid, 
but at a fairly low temperature, for a long time; that it has excellent 
opportunities for assimilating the wall rock; and that in each period of 
dormancy much of it passes through the stage of crystallization. Prog- 
ress in magmatic differentiation decidedly favors explosiveness, which 
is due to the tension of the gases, so that it is only in fairly mature vents 
that this feature of volcanic activity is pronounced. 
Direct offshoots of main abyssal injections have so far been con- 
sidered. They form batholiths; and plutonic stocks and bosses are inter- 
preted as cupolas in batholithic roofs. Sheets, dikes, and laccoliths are, 
however, distinctly satellitic, and soon lose thermal and hydrostatic 
communication with the main abyssal injection. It is only in the case 
of laccoliths that a mass of molten rock may be injected which will 
retain its heat for considerable periods; and such a mass may give rise 
