1884.] NATURAL SCIENCES OF PHILADELPHIA. 245 



less pure, being often colored by oxide of iron or organic matter, 

 and is much more thinly bedded, the thinner seams being inter- 

 stratified with the heavily bedded marble. 



Scrientine. — About six miles northeast of Wilmington a huge 

 dike of serpentine runs with the micaceous schists. Its length 

 can be traced by outcropping boulders for a distance of a mile, 

 with a width of a quarter of a mile. The rock varies from one 

 tough and massive to one soft and highly decomposed, with 

 which are associated talc and magnesite. 



Vitreous Quartz and Quartzite. — The former rock occurs as 

 regular thin or massive seams interstratified with the micaceous 

 rocks. It varies from a glassy colorless variety to one of milky 

 and opaque whiteness. The quartzite, which is found in the 

 northwest corner of the State, underlying the limestone, is 

 probably of Potsdam age. It is a very coarse quartzite rock, 

 which contains, frequently, crystals of tourmaline, Jibrolite and 

 actinolite. 



Structural Relations of the Crystalline Rocks. 



Strike and Dip. — The rocks of this formation, except in a very 

 few cases, are all stratified with variations of bedding, from that 

 as thin as slate in the mica schists to that so heavy as to resemble 

 massive trap intrusions. Both strike and dip are subject to 

 great variation. A dip to the southeast for a long distance, with 

 an opposite dip along a section only two miles and a half to the 

 northeast, can be accounted for only by supposing an unequal 

 thrust from the direction of the contorting force. In fact, out of 

 five different sections made across the Azoic belt at various 

 points of its length, no two showed the same arrangement of 

 strata — from which we conclude that the thrusting force must 

 have acted very unequally along the entire length of the belt, 

 sometimes merely tilting the strata ; again, standing them on 

 edge, and j-et again, completely overturning them ; in some 

 places pressing them into close folds for a part or the entire 

 length of the section, in others leaving gentle or abrupt anti- 

 clinal and synclinal folds. 



Both strike and dip are also found to be subject to variations 

 from the disturbing action of granitic intrusions. At Dixon's 

 spar quarry the change is from N. 55° E. to N. 22° E. At Pleasant 

 Hill a granitic rein cuts through the limestone, which causes a 



