THIRD HYPOTHESIS OF DERIVATION 469 



Eleventh avenue, reached over 2,000 feet. That near Morningside ave- 

 nue, which I have been able to stud}^ in most detail, extended, with 

 short interruptions, from West One hundred and eighth to One hundred 

 and twenty-third street, fully 3,000 feet, including Morningside park and 

 the adjoining heights, seamed with the dark sheets from end to end ; 

 hornblendic at the south, more biotitic toward the north, a good example 

 of the latter being shown in the park, near One hundred and twenty-third 

 street, at bottom of ascent of a path. 



A section of the beds from west to east across the island, from river 

 to river, at Ninety-seventh to Ninety-ninth street, Avas especially inter- 

 esting for repeated outcrops of the hornblendic rock. Beginning near 

 the Hudson river, in Riverside park, a contorted layer of black horn- 

 blende gneiss, 2 feet thick, in close association with pegmatite seams, lay 

 in micaceous granitoid gneiss ; strike north 41° to 51° east, 85° > south- 

 east. Going eastward over a broad synclinal fold to the opposite side 

 at Ninth avenue, this layer of hornblende gneiss, with about the same 

 thickness, was found rising with dip 60° > northwest. Following over 

 a narrow anticline of the prevailing micaceous gneiss to Eighth avenue, 

 the hornblende gneiss again appeared in two or three parallel layers, 

 each 1 to 6 inches in thickness and about a foot apart, in micaceous 

 granitoid gneiss ; dip 45° > southeast. Beyond this to the eastward the 

 absence of exposure of the hornblende gneiss within Central park was 

 due to the occupation of that part of the cross-section by the nearl}^ hori- 

 zontal layers of very micaceous gneiss, with mica schists, in the upper 

 part of a broad synclinal fold. At Fifth avenue a layer of black horn- 

 blende gneiss, 2 to 3 feet in thickne^ss, rose near the east side of the fold, 

 very nearly vertical. Continuing eastward toward Madison avenue, beds 

 of micaceous gneiss occurred on the east side of the syncline ; strike 

 north 33° east, 80° > northwest. Between Madison and Lexington ave- 

 nues then followed a series of four sharp folds in hard, gray quartzose 

 gneiss ; strike north 39° east. Each of these four folds inclosed a group 

 of three layers of fine-grained, compact, black hornblende gneiss, partly 

 epidotic. The layers were each about 1 foot thick and 2 to 3 feet apart. 

 In the anticline between Madison and Fourth avenues these la^^ers stood 

 nearly vertical. The three other folds between Fourth and Lexington 

 avenues were reversed, with axial planes inclined to the eastward. Be- 

 tween Lexington and Third avenues another anticline carried the horn- 

 blende gneiss on each side in groups of three la3^ers, each a few inches in 

 thickness. Then followed low ground, without outcrops, to the bank of 

 the East river at Hellgate, where the hornblende gneiss again appeared 

 along the shore in small islets in the river and beyond on the shore of 

 Long island at Hallett's cove. The many outcrops along this cross-sec- 



LXV— Bull. Geol. Soc. Am.. Vol, 14. 1902 



