392 JULIEN 



their extremities they melt away vaguely into the adjacent 

 gneiss, rich in both black and white micas, the former probably 

 indicating in large part the final absorption and disappearance, 

 to an unknown amount, of antecedent occluded masses of in- 

 truded diorite. The outcrops of diorite schist and of its distinct 

 biotitic derivatives, abundant as they are, afford therefore but a 

 small measure of the number of the original dikes. Dark 

 blotches of indeterminate outline seem to bear further testi- 

 mony to a vast dissemination of occluded igneous matter and so 

 to the conclusion that the Manhattan stratum was originally 

 seamed through and through by an enormous number of dikes 

 of gabbro, bronzite rock and pegmatite. 



Occlusions on Manhattan Island. — Several varieties of these 

 may be distinguished. 



Schist Fragments Within Dikes. — Portions of the schists or 

 gneisses themselves are sometimes found to have become sep- 

 arated and enclosed within the margins of intrusive pegmatite 

 dikes, perhaps displaying initial stages of absorption ; for ex- 

 ample on the knoll in Riverside Park, opposite West 83d Street. 



Much more important, as already suggested, have been the 

 results of interchange, in reverse order, from igneous intrusions 

 cut loose during diastrophic movements from connection with 

 their underlying magmatic sources, swallowed up and perma- 

 nently imprisoned within the invaded schists and there now 

 found in various stages of shearing, alteration and absorption. 



Pegmatite Occlusions. — Examples of the acid occlusions 

 have been already sufficiently described. The amount of peg- 

 matitic and quartzose matter thus introduced has been so great 

 that huge masses of saturated gneisses have been converted in 

 part or wholly into bedded granite, well shown still at Mt. Morris 

 Park, at north end of Central Park and on Morningside Heights. 



The earliest series of intrusions evidently consisted of rocks 

 of basic and ultra-basic composition, both of which have be- 

 come almost completely metamorphosed into two present forms. 



Quartz-diorite Schist After Gabbro. — The intercalated sheets 

 and lenses of this rock and its variants (hornblende schist, 

 hornblendic gneiss, biotitic gneiss, biotite schist) often charac- 



