GNEISSES IN THE HIGHLANDS OF NEW JERSEY 701 



through them in all directions, often running parallel to the banding, and 

 elsewhere cutting across it The alteration is due not only to the prox- 

 imity of the main mass of the batholith, but to the immense amount of granitic 

 material which occurs intruded through the series, sometimes in large masses, 

 but very frequently in thin bands which have found their way in between the 

 beds of the invaded limestone, changing it into amphibolite, and presenting a 

 typical instance of lit-par-lit injection. The granite, furthermore, not only 

 penetrates this amphibolite series, but floats off masses of it, which, in the 

 form of bands, streaks, and isolated shreds, are seen thickly scattered through 



the granite in the vicinity of the contact The separate fragments of 



amphibolite, where completely surrounded by the granite, while clearly nothing 

 more than masses of altered limestone, are rather harder, and have a more 

 granitized appearance than the rock which is still interstratified with the lime- 

 stone. 



From the various descriptions which have been cited it appears 

 that the type of intrusion to which the term lit-par-lit injection has 

 been applied presents evidences that the invasion of the magma 

 is preceded by the advance of a wave of metamorphism into the 

 wall-rock, by which the character and composition of the original 

 material are radically altered. By the deposition of magmatic 

 minerals and by the removal in solution of certain of the previous 

 constituents, the composition tends to approach that of the magma 

 itself, and when blocks of wall-rock are finally engulfed in the 

 magma their composition may be so changed that their assimilation 

 effects but little change in the composition of the latter. 



If this conception is well-grounded the zone of metamorphism 

 which surrounds areas of lit-par-lit injection is not to be considered 

 as wholly an after-effect of intrusion, due to expulsion of volatile 

 substances during the consolidation of the magmatic mass, but also 

 as due to a preliminary process, ultimately leading up to an invasion 

 of the magma and assimilation of the altered material. 



SUMMARY 



A description has been given of the structural relations observed 

 in a certain area of banded gneisses of pre-Cambrian age in Northern 

 New Jersey, where unusually favorable conditions for observation 

 have been found. The structures shown here are beheved to be 

 typical of those prevaihng over considerable areas in this portion 

 of the state. Evidence is given leading to the belief that the struc- 

 tures at this locality cannot well be attributed to the squeezing-out 



