R. A. Daly — Abyssal Igneous Injection. 



211 



Expansion of the eartKs outer shell as a factor in moun- 

 tain-building. — But there is good reason to believe that the 

 contraction theory, as usually stated, carries only half a truth. 

 Both advocates and opponents of the theory have generally 

 spoken of the collapsing surface shell as of nearly constant 

 circumference since the time of original crustification, except 

 for the amount of overthrust and overfold represented in 

 folded and faulted regions. This shell is, indeed, conceived as 

 itself having slightly shrunk through loss of heat during geo- 

 logical time, little or uo emphasis being placed on those 

 agencies which tend to counterbalance this shrinking and even 



Fig. 3. — Diagrammatic cross-section showing relation of mountain-building 

 to simultaneous and subsequent abyssal injection of magna. Scale and 

 symbols the same as in figure 2. a-b (heavy line) : the surface on which the 

 shell of compression shears over the shell of tension in the orogenic thrust- 

 movement. Two large batholiths of slightly different ages are represented ; 

 the mountain-building is supposed to have been just completed. The 

 igneous bodies injected and crystallized before the epoch of mountain- 

 building are not shown. 



produce a net expansion of the shell. Nevertheless, it seems 

 highly probable that the original shell has actually grown 

 larger instead of remaining sensibly of constant circumfer- 

 ence, and that a part, perhaps the greater part, of the circum- 

 ferential shortening observed in the world's mountain ranges 

 is due to this fact. 



The outer skin of the earth, including its overthrust por- 

 tions, may grow areally larger according to two different pro- 

 cesses. 



a. Local cavities produced in the shell of compression by 

 crustal readjustments may be rapidly filled with magma from 

 beneath or, more slowly, with vein matter deposited from cir- 

 culating waters. True magmatic injections, such as dikes, 

 laccoliths, " chonoliths,"* etc., and possibly a percentage of 

 batholithic irruptions represent just so much additional matter 

 squeezed into the shell. These wedges when solidified, like 

 all the countless mineral veins, aid in transmitting and increas- 

 ing the thrusts affecting the shell as it collapses on the shrink- 

 ing "nucleus" of the earth. This view has been clearly 



* Defined in Journal of Geology, vol. xiii, p. 498, 1905. 

 Am. Jour. Sci. — Fourth Series, Vol. XXII, No. 129. — September, 1906. 

 15 



