430 A. A. JULIEN — AMPHIBOLE SCHISTS OF ^[ANHATTAX ISLAND 



dictory features, testifying at once both to extreme plasticit}^ and ex- 

 treme rigidity in comparison with the inclosing gneisses. 



First, plication and corrugation of layers. The evidence of extreme 

 plasticity in this rock, during the general folding and kneading to which 

 the strata of the island have been subjected, is ver}^ marked!}' and fre- 

 quently shown, not onl}^ b}' numerous folds, zigzag crumpling and dis- 

 tortion of the beds, but by corrugation of the layers, even down to 

 thinnest laminae. Thus hornblendic beds may often be distinguished at 

 a distance, in outcrops or in section along street cuttings, both b}' con- 

 trast of their black color, laminated texture, and sharp margins and by 

 their wavy folds or convolutions, as at the outcrop near West One 

 hundred and nineteenth street (plate 61). On closer approach the 

 laminse may be found characterized by minute corrugations, three or 

 more to the centimeter — very beautiful on cross-section, where thin 

 alternations of quartz and epidote occur, as at the West One hundred 

 and thirty-fifth street outcrop. Such distortion of folds sometimes 

 reaches to the degree of pinching out into isolated, almost cylindrical, 

 corrugated masses, whose extremities ma}^ appear in cross-section like 

 the ends of columns. These may lie separated in long fluted rolls, 

 whose ends bear a rude resemblance to sections of petrified trees, as, for 

 example, at West One hundred and eighth street and Kinth avenue. 

 This comparison, I find, has been anticipated sixty years ago by that of 

 Dr L. D. Gale in regard to similar structure in hornblende gneiss at 

 about Eightieth street on Fourth avenue : 



" Columnar gneiss is seen in many places on the island, but in none more con- 

 spicuous than at the south entrance to the tunnel. The columnar structure of 

 gneiss only occurs where the mica is replaced by hornblende. There is not the 

 appearance of crystallization, as in basalt and greenstone, but the fragments have 

 the appearance of old logs, or like the half or quarter of a log, as if split and quar- 

 tered by art." 



He further refers to the " columnar structure " in the hornblende 

 gneiss at Seventh avenue, south of McCombs bridge on the Harlem 

 river.* Similar remarkable flexures and isolated rolls have been de 

 scribed and figured by Dana from outcrops on the north side of West 

 One hundred and tenth street between Ninth and Tenth avenues (which 

 is still to be seen at northwest corner of One hundred and tenth street 

 and Morningside avenue west) and at West One hundred and thirty- 

 third to One hundred and thirty- eighth street between Tenth and Elev- 

 enth avenues. 



* Mather, op. cit., pp. 593, 599. 



