BASAL GNEISSES OF THE HIGHLANDS 365 



occur, and therefore the association is somewhat obscure, 

 although the relationship can be seen on the cliffs facing the 

 main channel of the river. Below Fort Montgomery, on the 

 contrary, great angular masses 20 to 50 feet in length, com- 

 monly of the quartzose and schist types and even the inter- 

 bedded limestones themselves are included in the granite in the 

 most strikingly clear-cut way. Nowhere are they better shown 

 than along the cuts of the West Shore Railroad at that place. 

 At the Mohegan quarries, the freshest and youngest and most 

 massive type of granite of the Highland region breaks through 

 not only the gneisses but also the uppermost of the crystalline 

 formations of the district. In the quarry and the adjacent hill- 

 side, large inclusions of gneisses, quartzites and schists occur 

 in even more complex development than in the other cases 

 mentioned. Occasionally fragments of the later formations lie 

 in proximity to those of the gneisses, all completely surrounded 

 by true granite matrix. 



With the sheetlike intrusions and smaller injections there is 

 much less disturbance of this sort, and the eruptive nature of 

 the contacts is chiefly evident in lenslike enlargements or dike- 

 like development. On a large scale this is best developed with 

 the " Yonkers gneiss," noted before, in the ridge extending from 

 Mount Vernon through Scarsdale and White Plains. Similar 

 relations on a large scale prevail in certain parts of the broad 

 gneiss belt nearest the Hudson river from Spuyten Duyvil to 

 the Croton valley. But in this case the intrusive is less easily 

 distinguished from the inclosing series ; it is itself gneissoid, 

 and is more irregular in areal development, small intrusions of 

 similar type are exceedingly numerous, too numerous and too 

 limited in extent to be mapped or indicated separately. The 

 lower limit is represented by the small pegmatite injections of 

 a few feet or rods in length, lenslike bunches and stringers, 

 sometimes making up a considerable portion of the formation, 

 in some places surely connected with larger igneous masses, 

 while in others they may have no true igneous origin. All 

 are essentially but parts of the great basal gneiss, and the 

 line separating those of sufficient prominence to be mapped as 

 distinct members from those neglected or merged in the general 

 color is wholly an arbitrary one. 



