246 DEPARTMENT OF T>HE INTERIOR 



2 GEORGE V., A. 1912 



magma. The product of this digestion is not now evenly disseminated through 

 the crystallized gabbro, which, except near its upper and lower contacts, is very 

 nearly identical in composition with the unacidified gabbro occurring elsewhere 

 in the district. No conclusion seems more probable than that the material of 

 the dissolved blocks is now for the most part resident in the acid zone at the 

 upper contact. The same view holds for the perhaps much more voluminous 

 material dissolved by the magma at the main contacts themselves. The excess 

 of acid material at the lower contact was held there because of the viscosity 

 of the magma in its final, cooling stage. For the greater bulk of the digested 

 material there, has been, it appears, a vertical transfer upwards, a continuous 

 cleansing of the foreign material from the basic magma. 



Assimilation at Deeper Levels. — Another cause of acidification is to be 

 sought in the conditions of sill-injection. In the Purcell range, as generally 

 throughout the world, channels (dike-fissures) through which the magmas have 

 been forced into the greater sill chambers, are relatively narrow as compared 

 with the thickness of the respective sills. In most cases the feeding fissures 

 seem also to be few in number for each sill. The magma must pass through 

 such a fissure during a considerable period of time in order to form the 

 enormous bulk of a first-class sill. At' that stage the magma is at its hottest 

 and it is being moved rapidly past the country rock. The effect is analogous 

 to that of stirring a mixture of salt and water; solution is stimulated by the 

 movement. The original magma is thus converted into ' a syntectic magma, 

 with greater or less chemical contrast to the original. 



Such a case may be represented in the great sill, in Xew Jersey, which 

 outcrops for a distance of more than a hundred miles. Lewis has shown that 

 its rock is chiefly a quartz diabase.' 55 ' Since the sill-rock shows chilled contacts, 

 it appears probable that its magma after reaching the sill-chamber, was too cool 

 to accomplish much solution on roof or floor, though some xenolithic material 

 (sandstone) may have been dissolved. The special composition and structure 

 of the New Jersey sill can be explained as that of a syntectic of primary 

 basaltic magma, which dissolved a small proportion of the acid rocks (sand- 

 stones and pre-Cambrian crystallines) forming the walls of the feeding fissures. 

 Though the temperature of this sill was too low for much evident solution of 

 the sill's floor and roof, it was high enough, and the sill magma therefore fluid 

 enough, to permit of the remarkable gravitative differentiation described by 

 Lewis. 



Some acidification of the Purcell gabbro may. thus, have occurred in its 

 long passage through the thick lower quartzite? and other sediments of the 

 Purcell series. Nevertheless, the great chemical similarity between the biotite 

 granite and the average quartzite strongly suggests that the assimilation in the 

 Moyie-sill magma chiefly occurred in the quartzite formation, and not in the 

 underlying pre-Cambrian formations of differing composition. 



* J. V. Lewis, Annual Report, State Geologist of Xew Jersey, 1907, p. 99. 



