R. A. Daly — Mechanics of Igneous Intrusion. Ill 



causes, the energy may be derived from the necessary expan- 

 sion of volume suffered by the digestion of solid country-rock 

 by the magma.* The development of the fissures now filled 

 with apophysal material may, however, be powerfully aided 

 by a third cause that controls as well the formation of the 

 other of the two zones noted above. It may be believed that 

 a common disrupting force has affected both zones ; a brief 

 description of the second zone may anticipate the analysis of 

 the conditions which demand the energetic working of that 

 force. 



The Zone of Inclusions ; its origin. — The second zone is 

 composed of igneous rock enclosing blocks of the country-rock. 

 As the apophyses, breaking the continuity of the invaded for- 

 mation, vary enormously in number within the outer zone, so 

 the blocks, breaking the continuity of the igneous body, show 

 the greatest variation in tlie degree of their abundance. This 

 "zone of inclusions" varies in width from a few meters to 

 three kilometers or more. The blocks, unless very close 

 together and possessing thoroughly massive structure them- 

 selves, usually show clear evidence of having been shifted out 

 of their former relative positions in the invaded formation, so 

 that their original orientation is completely lost. There are 

 transitions to the outer zone through the gradual increase in 

 the number of blocks left undisturbed from their original 

 orientation ; and there is, of course, no easily fixed boundary 

 between the zone of inclusions and the main intrusive body in 

 which country-rock inclusions are normally absent or very rare. 

 The inner boundary of the zone of inclusions is often difficult 

 to determine in the case of stock or batholith so exposed to 

 view by denudation as to furnish a land-surface close to the 

 former roof of the magma-chamber. 



Whatever be the causes of the disruption of blocks now 

 found in the zone of inclusions, those causes are directly con- 

 nected with the intrusive body itself and are thus not external. 

 The zone is, for example, not due in the normal case to the 

 injection of magma into rock coarsely brecciated by regional 

 dynamic movements in the earth's crust. Movements of that 

 sort tend generally to brecciate rock along straight or open- 

 curve lines and would not necessarily follow the complex, 

 sinuous, closed-curve line of contact such as belongs to a plu- 

 tonic body. There is certainly, on the other hand, a genetic 

 relation between the zone of inclusions and the replacement of 

 the country-rock by great bodies of intruded magma almost or 

 quite free of foreign fragments. Many authors speak of the 

 inclusions as having been " torn off " or " carried up " by the 

 * This Journal, April, 1903, p. 289. 



