I 



QUARRY MATERIALS OF NEW YORK I33. 



than the Yonkers, owing to the larger proportion of biotite. The 

 texture indines to finely granular, except when injected by coarse 

 granite. 



With respect to the other gneisses and igneous rocks of this 

 section, the Fordham occupies a basal position, so that its early 

 Precambric age seems established. It is clearly intruded by the 

 Yonkers. As it is made up largely of sedimentary material, it may 

 be classed as Grenville, which is the position assigned to it by 

 Berkey. 



The sedimentary derivation of the gneiss is strongly suggested 

 by the regularity and persistence of the banded structure, which 

 resembles true stratification. Further evidence of this origin is 

 found in the gradation into quartzite that is observable in places, 

 and also by the bands, streaks and irregular masses of calcareous 

 material which are included within the formation. These inclusions 

 become of considerable importance in the northern extent of the 

 Fordham and are seen not infrequently in Westchester county. 

 The banding of the gneiss is referable in greater part to variations 

 in the original sediments which are believed to have been of the 

 nature of impure limestones, shales and shaly sandstones. 



Granitic and pegmatitic injections have taken place in parts of 

 the Fordham along the planes of foliation. The igneous material 

 may form thin bands or veins that alternate more or less regularly 

 with the gneiss, with sharp contacts ; or it may impregnate the 

 body of the gneiss itself. Occasional dikes of these rocks cut across 

 the bedding. 



Physical character and composition. The gneiss is medium to 

 dark gray in color, with a pinkish tone when there is much granitic 

 mixture. The banding is its most striking feature. By reason of 

 the parallel arrangement of the biotite, the dark bands partake of 

 a certain degree of schistosity, cleaving or breaking rather readily 

 along the foliation. The most persistent joints follow the foliation. 

 These are variably spaced, from a few inches apart, where the 

 gneiss has been crumpled or shattered, to several feet in the un- 

 contorted rock. 



The texture of the gneiss is fairly even but extremely fine. The 

 diameter of the feldspar particles ranges from .25 to 3 mm, and 

 the quartz is only slightly larger. The feldspar in most places shows 

 incipient kaolinization, but otherwise there is little alteration notice- 

 able. The biotite is somewhat bleached and the iron set free is 

 segregated in the cracks and sutures. Muscovite and hornblende 

 are usually present in small amounts. 



