ROGERS, GEOLOGY OF THE CORTLANDT SERIES 47 



several occurrences of serpentine. These are probably generally, if not 

 always, dikes and they will be described under that head. They always 

 occur in the pyroxenite area, however, and may represent local segrega- 

 tions of olivine. They would thus once have been dunites, since only 

 serpentine, showing mesh-structure, is contained in them. If they are 

 genetically related to the peridotites, however, it is difficult to under- 

 stand their complete alteration ; and while their field relations cannot be 

 definitely established, they may be provisionally considered as dikes. 



DIKE ROCKS 



Dikes are fairly common through the district and are usually small; 

 and the following list is therefore probably by no means complete. Aside 

 from those within the area itself, an excellent development of chiefly 

 basic varieties occurs on Verplanck Point, wjiere the dikes ramify into 

 the limestone and schist. The abundance of pegmatite dikes in the north- 

 ern part of the district has already been remarked. The complex of rocks 

 on Montrose Point, in which, as Williams says, "each rock includes and 

 forms dikes in every other" is interpreted as a case of true differentiation, 

 rather than as a network of dikes; and the rocks have therefore been 

 considered above. A careful study of the dikes of the series with regard 

 to their mutual time relations would probably be of great value in indi- 

 cating the relations of the plutonic types; for if the dikes had followed 

 the laws which govern their successive differentiation in other districts, 

 it might be inferred that the plutonics had done the same. 



Aplite 



The aplite dikes are usually small and not especially common. They 

 occur chiefly north of Pleasantside. They are, of course, fine grained and 

 consist of muscovite, orthoclase and quartz, with small amounts of plagio- 

 clase and biotite and with zircon and magnetite as the common acces- 

 sories. They are, perhaps, slightly more acidic than the granites, but 

 their composition does not differ greatly from the latter. 



Pegmatite 



Pegmatite is probably the most common dike rock of all. It was no- 

 ticed especially in narrow veins or dikes in many of the more northerly 

 emery cuttings. The pegmatitic structure is generally well developed, and 

 the component minerals are as usual. The largest and most striking 

 occurrence of the rock appears about 200 feet south of Montrose station, 



