GNEISSES IN THE HIGHLANDS OF NEW JERSEY 697 



It is true that the larger bodies of granite show a tendency toward 

 a lenticular outhne, indicating that in such instances the adjacent 

 rock has been forced apart; and it is found also that flexures of a 

 minor order have been produced in the layers, but the striking 

 fact remains that frequently evidences of the original structure 

 survive in the midst of large masses of granite and show little dis- 

 turbance of position. The most noticeable evidence in this respect 

 is the parallelism shown by the bands of inclusions with the regional 

 strike of the gneisses. This is observable even in those cases where 

 assimilation by the magma has proceeded so far that the included 

 material no longer appears as distinct bands but merely as dark- 

 colored schlieren of indeimite outline. Moreover, the presence 

 of faint parallel lines of dark minerals within the granite along- 

 side of inclusions, as well as a certain parallelism visible in the 

 arrangement of the component minerals of granitic masses in 

 which no distinct xenoliths are perceptible, implies that some sort 

 of structural framework of solid material was retained. It cannot 

 be supposed, therefore, that the injection of the magma was of a 

 sudden and violent nature or that it entered by simply wedging the 

 original rock apart along the dividing laminae and occupying the 

 spaces made. On the contrary, it must have gained access in such 

 a slow and quiet manner that in many places delicate structural 

 relations were left undisturbed. In fact the process appears to have 

 possessed many of the features of a gradual substitution rather than 

 the violent characteristics of intrusion as we often picture them. 

 The advocates of lit-par-lit injection to whom reference has 

 been made have supported the theory that the advance of the main 

 body of magma was preceded by that of a more attenuated portion. 

 They beheve that these solutions in advance, which were probably 

 similar in composition to that of the main body but characterized 

 by the presence of a larger percentage of the volatile ingredients, 

 exercised functions of a preparatory character, metamorphosing 

 the material into which they penetrated and rendering it gradually 

 more susceptible to the action of the magma which followed. La- 

 croix's conception^ may be illustrated by the following quotation, 



^A. Lacroix, "Le granite des Pyrenees et ses phenomenes de contact," 

 Carte geol. de France, No. 7/ (1900), p. 26. 



