6io CLARENCE N. FENNER 



observed in certain areas of pre-Cambrian rocks of Fenno-Scandia 

 may be applicable. He urges that in certain cases the strata have 

 been buried to such a depth that the rocks as a whole have become 

 plastic, and the more fusible portions have actually formed a magma 

 at the horizon observed or at a comparatively small depth below. 

 We are hardly warranted from the evidence at hand in accepting 

 this explanation here. The general parallelism and straightness 

 of the bands indicate that prior to the injection of magma the 

 dominant characteristic of the rock was fissility rather than plas- 

 ticity. The phenomena of softening must therefore be attributed 

 to the injection itself, but the evidence shows that while the magma 

 was still in movement softening had ensued, and we are led to con- 

 clude that the process of injection was long continued. It appears, 

 however, that in the New Jersey Highlands large areas show evi- 

 dences of granitic injection of the type which has been described, 

 suggesting that beneath such areas the conditions necessary for the 

 softening and flow of masses of rock may have been realized. 



The process by which the granite and gneisses have assumed the 

 relations described in the foregoing pages appears to be similar 

 in many respects to that which French geologists, notably Michel- 

 Levy and Lacroix,' have termed lit-par-lit injection. In their 

 typical examples the phenomena are attendant upon the intrusion 

 of batholiths of igneous rock into areas of Paleozoic sediments, and 

 the contact phenomena may be plainly seen. In the New Jersey 

 example the appearances imply rather the presence of a reservoir 

 of magma at some lower level, from which offshoots in large and 

 small apophyses were injected into the roof and are now exposed 

 for observation. In many respects, however, the mode of injection 

 appears so similar that the same term may be apphed. 



It will readily be seen that the resultant gneisses are believed 

 to be composite rocks, made up of material of different sorts, derived 

 from different sources. An older, sheared or bedded rock is postu- 

 lated, to which magmatic material has been contributed; first, by 



' The publications of these two authors which bear upon the subject are quite 

 numerous. Special mention may be made of Michel-Levy's Bull. No. 36, Carte 

 geol. Jran. (1893), and Lacroix's Bull. No. 64 (1898) and BiM. No. 71 (1900), 

 Carte geol. fran. 



