40 JR. A. Daly — Mechanics of Igneous Intrusion. 



difficulty of understanding how plntonic magma, which is 

 capable of intrusion, can become superheated more than two 

 or three hundred degrees Centigrade. 



Fourthly, the stoping hypothesis has the special advantage 

 of providing a mechanism of thorough agitation within a bath- 

 olith. Strong stirring of the mass is induced by the sinking 

 of xenoliths and by the necessary rising of the magma locally 

 acidified by their solution. This agitation can explain the 

 marvelous homogeneity in each large batholith. It helps 

 greatly to explain the manifest evidences of magmatic differ- 

 entiation within batholiths — splittings and segregations that 

 cannot be due to the slow process of molecular diffusion or to 

 mere thermal convection. The whole process of stoping and 

 the rising of syntectic magma tends to equalize the temper- 

 atures in the batholithic chamber and thereby we can under- 

 stand the even grain and rapid, nearly simultaneous crystalli- 

 zation of a batholith throughout its visible depth. 



Fifthly, the engulfment of blocks of geosynclinal sediments 

 enriches all parts of the batholiths with water, chlorides, etc. 

 which so greatly aid solution ; while, on the older view, these 

 agents are confined to the uppermost part of the chamber. 



Sixthly, as already noted, the cleansing of syntectic films 

 from contact of solid and liquid is much the more rapid and 

 perfect according to the stoping hypothesis, thus providing 

 and renewing conditions for molecular lowering of the fusion- 

 point along contacts. 



In short, the newer view has the advantage of not only 

 better explaining the facts of the held but it is incomparably 

 more economical of the heat postulated for the work of bath- 

 olithic replacement than is the theory of pure marginal assim- 

 ilation. Melting and marginal assimilation of country-rock 

 takes place in the initial, superheated condition of a basaltic 

 injection, but must be regarded as always subordinate in replace- 

 ment efficiency to stoping and abyssal assimilation. 



Existence of basic stocks and batholiths. — Finally, the fact 

 that some large bodies of plntonic rocks are basic has been 

 advanced as an objection against the idea of stoping.* This 

 fact early impressed itself on the present writer and led to his 

 reviewing the geological literature to determine, if possible, 

 the number, distribution, and age of these bodies. It was 

 found that most of those which have undoubtedly batholithic 

 development on a large scale are of pre-Cambrian age and 

 are chiefly anorthosite intrusions. In this Journal, vol. xx, 

 1905, p. 216, the guarded suggestion was made that the anor- 

 thosites of Canada and the Adirondack Mountains are so basic 

 because of the absorption of crystalline limestones. On more 

 * W. Cross in Science, xxv, p. 620, 1907. 



