388 JULIEN 



Delaware 431 



Pennsylvania 433 



Massachusetts 435 



Location of cities along the occlusion line 436 



Coastal chain of volcanoes 437 



Mount Manhattan and its associated peaks 438 



The terms incbision and enclosure have been commonly ap- 

 plied to fragments of foreign rocks imbedded within sedimentary- 

 deposits, crystalline schists or masses of igneous intrusion. 

 Thus, to take illustrations from structural features on Manhattan 

 Island, New York City, we find here ordinary dikes of pegma- 

 tite and aplite included in the upper stratum of micaceous 

 gneisses and schists, which they frequently intersect, running 

 partly along the foliation. Tongues of the schist itself 

 also, or, it may be, lenses and sheets between the crossing 

 dikes, project into the larger masses of intrusive granite in a 

 manner fitly pictured by the broad term, inclusion. The same 

 hard-worked term has also acquired a restriction, in optical 

 mineralogy and lithology, to microscopic enclosures within min- 

 erals and rocks. 



Need of the Term, Occlusion. — For foreign masses, com- 

 pletely engulfed and enwrapped, whose constituents are con- 

 sequently in state of reaction and interchange wdth those of the 

 surrounding country rock and of general absorption into it, there 

 seems to be call for another term which may specifically define 

 relationship to a new stratigraphical unit. Our language is 

 surely for common service. Though the word occlusion has 

 long been in use by our brother physicists and chemists, with 

 reference to envelopment and absorption of fluids, may not the 

 geologist safely borrow it, without danger of confusion, in appli- 

 cation to somewhat allied phenomena found in his own field ? 

 Its use will tend, I think, toward clearer recognition of the con- 

 stant passage, alteration, absorption and disappearance of masses 

 of imprisoned rock into a surrounding stratum and of extensive 

 metamorphic changes thereby effected. 



Occlusions in Igneous Rocks. — Attention has been directed 

 toward the instances of envelopment of rocks of all classes 

 within igneous dikes and flows and the consequent alteration, 



