"728 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



"to the intrusion. Both belts lie parallel to the average line of contact between 

 the intrusive and the country-rock. 



The belt more remote from the intrusive body is generally much the broader 

 of the two and consists of country-rock intersected by more or less numerous 

 apophyses from the main igneous mass. 



The second belt is composed of igneous rock enclosing blocks of the country- 

 rock. As the apophyses, breaking the continuity of the invaded formation, 

 vary enormously in number within the outer belt, so the blocks, 

 breaking the continuity of the igneous body, show the greatest variation in 

 abundance. This belt of inclusions varies in width from a few feet to 

 two miles or more. The blocks, unless very close together and possessing 

 thoroughly massive structure themselves, 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 belt 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 belt of inclusions and the main intrusive body in which country- 

 rock inclusions are normally absent or very rare. The inner boundary of the 

 belt 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 belt 

 of inclusions, those causes are directly connected with the intrusive body itself 

 and are thus not external. The belt 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 brec- 

 ciate 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 plutonic 

 body. There is certainly, on the other hand, a genetic relation between the belt 

 cf 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 ascending magma, 

 without, however, showing the possibility of such a process when correlated with 

 the apparently demonstrated liquidity of plutonic magmas. 



Some of the blocks within the belt 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 anas- 

 tomosing apophyses. But there are reasons for concluding that apophyses 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* 



* These and many following paragraphs are adapted from the writer's papers on 

 ' The Mechanics of Igneou6 Intrusion/ American Jour. Science, Vols. 15 and 16, 1903, 

 and Vol. 26, 1908. 



