108 B. A. Daly — Mechanics of Igneous Intrusion. 



In the present communication certain other concrete illustra- 

 tions will be offered along with additional discussion of some 

 of the main premises on which the newer hypothesis is based. 

 Among these premises are : (1) the generally high fluidity of 

 igneous magmas during the time of active intrusion ; (2) the 

 normally extensive shattering of the invaded formations at 

 their contacts with plutonic magmas ; and (3) the reality of 

 " overhead stoping " of the blocks so shattered and rifted off 

 from wall or roof of the chamber. Concerning the first and 

 last of these premises certain difficulties were hinted at or par- 

 tially discussed in the former paper, but their importance 

 demands that further attention be paid to them. A note on a 

 particular point referring to the first-mentioned premise may 

 well anticipate the larger division of this paper, which will 

 have to do primarily with contact-shattering in general and 

 with the testimony of several Canadian intrusive bodies as to 

 the validity of the rifting and stoping hypothesis. 



The origin and suspension of hasic segregations : hearing 

 on the doctrine of the liquidity of i^lutonic magmas. — At first 

 sight, the common occurrence of basic segregations freely sus- 

 pended in their rock-matrix always less dense than themselves 

 seems in one way to militate against the idea of high fluidity 

 in igneous magmas. It is true that the mere fact of that par- 

 ticular kind of differentiation calls for the assumption of no 

 inconsiderable degree of mobility in the igneous mass ; but the 

 question arises as to how that substance now represented by 

 heavy minerals could be segregated and held up in a less dense 

 matrix possessing such liquidity. That segregations have been 

 so suspended seems clear in the average field occurrence. The 

 question raises a second one as to the relative densities of 

 molten segregation and molten matrix. If it can be shown 

 that in the fluid state, matrix and segregation are nearly of the 

 same density, the difficulty disappears and therewith the objec- 

 tion to the theory of high liquidity. 



While the variations in thermal expansion of the common 

 plutonic rocks are confined to narrow limits, the experiments 

 so far made on the volumetric increase observed in the passage 

 of single crystalline minerals to the glassy condition (at room 

 temperature) appear to show that the law of even approxi- 

 mately constant ratio of expansion in rock-forming minerals 

 does not hold. The following table taken from Zirkel's Lehr- 

 buch der Petrographie (1893, vol. i, p. 681) synopsizes the 

 results of Deville, Thoulet and others. The percentage 

 decrease in density suffered by the minerals investigated when 

 these pass from the crystalline state to that of cold glass is as 

 follows : 



