112 H. A. Daly — Mechanics of Igneous Intrusion. 



ascending magma, without, however, showing the possibility of 

 such a process when correlated with the aj^parently demon- 

 strated fact of the high liquidity of plutonic magmas. On the 

 assumption of high liquidity at the moment of the immersion 

 of the blocks, they would, in the average case, not remain near 

 the molar contact,* but would sink into the depths of the 

 magma-chamber. It is further imj^ossible to believe that any 

 kind of current action in the magma could carry on a sort of 

 erosion on the chamber-walls, a hypothesis which likewise 

 would fail to explain the constant relation of the zone of 

 inclusions to the molar contact. 



Some of the blocks within the zone of inclusions have 

 unquestionably been floated out or sunk from the molar contact 

 after those portions of the country-rock have been completely 

 surrounded by magma of the main body and of anastomosing 

 apophyses. But there are reasons for concluding that apoph- 

 yses of an abundance, matching the countless inclusions of 

 many internal contact-belts, were not formed simply by reason 

 of hydrostatic pressure forcing magma into original cracks or 

 fissures in the country-rock. The conditions reigning at the 

 contact imply the exhibition of a different source of energy — 

 one which many geologists have incidentally credited with the 

 shattering effect. A clear, positive statement of the case has 

 been given by Crosby in his monograph on the Blue Hills 

 Complex. f The subject is of importance and merits much 

 more discussion than can be brought into the few pages. of this 

 paper ; it is, moreover, a difficult subject, largely on account of 

 the existing lack of experimental data, and the following treat- 

 ment of it- can lay claim to doing little more than open up the 

 problem and make the attack upon it in a qualitative manner. 



Shattering hy differential thermal expansion in the invaded 

 formation. — It is manifestly impossible to determine the exact 

 rise of temperature which w^ill occur in a formation at the con- 

 tact with an invading magma. Both elements, the pre-eruption 

 temperature of the country-rock and the temperature of the 

 magma itself, are partly indeterminate. If the former be regu- 

 lated by the normal law of the vertical distribution of the 

 isogeotherms, that temperature will be about 200° C at a depth 

 of four miles below the earth's surface — possibly a rather 

 liberally estimated average depth for the upper limit of a 

 granitic magma-chamber. If we assume that the temperature 

 of an intruding magma is approximately that at which the rock 

 resulting from its crystallization becomes thinly molten under 

 plutonic pressures (an assumption apparently justifiable from 



* The main contact, that of igneous body ')nass against country -rock mass, 

 as distinguished from the minor contact of inclusion and intrusive rock, 

 f Occasional Papers, Boston Soc. Nat. Hist., iv, 1900, p. 315. 



