202 NEW YORK STATE MUSEUM 



3 Fordham gneiss (bottom), the usual black and white thinly 

 banded type, a much folded and strongly metamorphosed rock, the 

 oldest of all. 



4 Yonkers gneiss, the usual type, gneissoid biotite granite very 

 uniform and granular. This formation is an igneous intrusive that 

 cuts up through the Fordham gneiss and is therefore younger. 

 Whether it is also younger than the limestone and schist is not 

 clear. 



5 Quartz veins and lenslike segregations of quartz, also pegma- 

 titic streaks, are occasional occurrences in all of the formations. 

 They are most abundant in the schist, but are seen also in the Ford- 

 ham gneiss. A similar development was encountered in the lime- 

 stone in hole no. 40. 



6 Glacial drift, ohiefly modified drift, partially stratified sand 

 and gravel, reaching more than 125 feet in depth, covers por- 

 tions of all formations. 



This last formation (no. 6) is the only one that may be wholly 

 avoided in the tunnel proper. The chief interest lies in its hindrance 

 to exploration and its possible usefulness as a source of sand and 

 gravel supply. 



Weakest formation. The Inwood limestone is the most ques- 

 tionable ground. This is believed to be so chiefly because of the 

 greater solubility of the rock, its granular and micaceous character, 

 and the probability that a line of displacement accompanied by some 

 fracturing crosses the siphon line in this formation. If a very 

 excessive amount of shattering occurs in this zone it may have 

 induced a condition of disintegration to such depth as to endanger 

 the tunnel. 



There are no surface indications of a serious condition at depth 

 for any of the other formations. 



Critical zone 



The critical zone is probably not far from the contact between 

 gneiss and limestone. There are two reasons for this opinion. The 

 first is related to the nature of the folding. The formations are 

 squeezed into a close syncline pitching northward. In cross section 

 the strata at any point around the head of this trough dip inward, 

 and, because of the more resistant Fordham gneiss forming the floor 

 of the trough, the drainage and seepage and consequent tendency to 

 decay might be expected to follow along its upper contact. 



