THIRD HYPOTHESIS OF DERIVATION 



473 



5 layers at West One hundred and eighty-seventh street and Wadsworth 

 avenue ; 



13 layers on the Speedway about West One hundred and ninety-third street ; 

 5 layers on the Speedway near Dyckman street, etcetera. 



The explanation of this local concentration of the hornblende schist in 

 ])arallel sheets in close proximity may be connected with the method and 

 the time at which their injection into the strata took place. If this had 

 occurred after the compression of the gneisses into their general sharp 

 folds by upward thrust along the more easily parted division planes, 

 there would have been no tendency to close concentration of parallel 

 sheets, and, amid the large number of outcrops of more gentle folds, little 

 possibility of entire absence of intersection ; of this the later system of 



Figure 8. -Diagrauimatic Sketches illustrating two Periods in History of a Dike ivith i.ts Apophyses, and 



first Effect of Shearing. 



Supposed to precede the results shown in phite 61. 



acid or pegmatite dikes is a proof. Obviously, then, I think, their in- 

 trusion has preceded the folding of the strata, and this close grouping of 

 sheets of hornblende schist may be the result of crowding down of clusters 

 of apoph3^ses whose line of union, the main dike or pipe, has thus be- 

 come distorted, broken up, and effaced. Perhaps the finest example of 

 this is shown at the outcrop on the Speedway (opposite Durands', about 

 One hundred and ninety-third street), where a cross-section of 12 feet 

 comprises 13 layers of hornblende schist, from l2^ to 20 inches thick. In 

 such a mass of contorted layers, all parallel (as also in those at West One 

 hundred and thirty-fifth street, near Eleventh avenue; West One hun- 

 dred and sixty-fifth street, near Edgecomb avenue, etcetera), the plication 

 may be safely attributed to lateral compression during folding of the 

 stratum. 



But in other cases the convolutions in a single la3^er of schist — for 

 example, at West One hundred and twenty-fifth street near Claremont 



