OCCLUSION OF IGNEOUS ROCK 391 



basic schists, usually attended, first, by development of columnar 

 or even fibrous texture under much crushing, associated with 

 symmetrical jointage, and, ultimately, with a granular texture. 

 Under extreme conditions, the diorite schist may become rolled 

 up into isolated cylindrical masses, and, it is probable, some- 

 times concentrated by thickening, in the direction of the pressure. 

 Good examples of these phenomena have been also noted in 

 occluded masses of diorite, passing into hornblende schist, inter- 

 calated in limestone parallel to its foliation, at Jenny Jump 

 Mountain in northwestern New Jersey, concerning which it is 

 stated : 



" In some cases it has been seen that these eruptiv^es at a 

 short distance from the contact are banded, and these banded 

 phases, if seen alone, would not be distinguishable from the 

 ordinary banded hornblende gneisses of the region." ' 



Squeezing of a plastic occluded mass outwardly from the 

 margin of a lens along foliation planes of the adjoining country 

 rock. Thus we find our diorite schist often intercalated in sev- 

 eral or many thin sheets projected between parting planes of the 

 gneiss in the same stratum, in forms which may be termed sec- 

 ondary or occlusion-dikes. These simulate apophyses of ordi- 

 nary or primary dikes and even show a limited tendency to in- 

 tersect thin layers of the gneiss.^ 



Chemical Processes Attending Occlusion. — These vary largely 

 with the relative composition of enclosed igneous material and 

 of contiguous rock. On Manhattan Island the pegmatite seams 

 and lenses are bordered with excess of quartz, often intermixed 

 with tourmaline, apatite, micas, and microcline. At the margin 

 of the basic rocks, the diorite schists, the reactions are still 

 more marked. The hornblende disappears by alteration to 

 biotite, the rock becoming interlaminated with or entirely 

 changed into biotitic gneiss or even biotite schist, with more or 

 less garnet. These layers form gray bands, with the slaty 

 lamination, sharply defined boundaries and often zigzag corru- 

 gation, characteristic of the original hornblendic layers. Toward 



'J. E. Wolff, Rep. State Geol. N. J., 1895, 52-59. 

 2Julien, loc. cit., Plate 63, Fig. i. 



