Pirsson — Petrographic Province of Central Montana. 47 



modus operandi on a vast scale. But within limits and with 

 sufficient time it may be a factor of importance, and com- 

 bined with convection currents and forced movements on a 

 small scale due to the passage of heated gases, and on a large 

 one to dynamic movements of the crust, a variety of agencies 

 may be brought into play which may be sufficient to render a 

 body of homogeneous magma, even if of considerable size, quite 

 diverse in its parts. And it is to be clearly noted that this is 

 quite independent of the question as to whether the magma 

 shell of the molten earth (supposing there ever was such a thing) 

 was ever homogeneous or not. This is purely an academic 

 question and may always, as at present, remain a matter of 

 mere speculation. Whether it was or not, the magmas under- 

 lying the crust are different now in different regions, and this 

 is the basic fact with which the petrographer has to concern 

 himself. 



On the other hand, it is the writer's belief that it would be 

 unreasonable to throw away such indications as those afforded 

 above, that the distribution and occurrence of igneous rocks are 

 not due to mere chance ; to deny they are governed like other 

 things in nature by definite laws and processes ; to affirm that 

 they are caused by mere haphazard heterogeneity of the under- 

 lying magma, and to thus dispose of the subject by relegating it 

 to chaos. 



In regard to local as distinguished from regional differen- 

 tiation, we know something of the conditions and occurrences 

 most favorable for its operation of the magmas in which it is 

 most likely to occur and to predict some of the probable results 

 of its operation. But in regard to the latter it does not seem 

 to the writer quite reasonable to assume that agencies and pro- 

 cesses that would be operative on a small scale would be nec- 

 essarily applicable to vast bodies of magma underlying great 

 regions. The writer has already discussed this phase of the 

 subject in another place and it is not necessary to repeat it 

 here.* But in the writings and speculations of many petrog- 

 raphers a good deal of confusion on this subject appears and 

 the differentiation of huge " magma basins " presumably cover- 

 ing hundreds and even thousands of square miles, is discussed 

 in the same terms and with appeal to the same supposed 

 agencies as has produced visible results in a single dike, lacco- 

 lith or other relatively small rock body. In the writer's opinion 

 this is wrong and will only tend to throw discredit upon what 

 has so far been produced that is of real value. It must be 

 confessed that at the present time so little is known and so 

 much remains to be discovered that any attempt, from the 



- Igneous Eocks of the Highwood Mountains, Bull. 237 U. S. Geol. Surv., 

 p. 183. 



