246 PROCEEDINGS OP THE ACADEMY OF [1884, 



disturbance from N. 55° E. to N. 10° E. Excluding cases of local 

 distortion, however, the crystalline rocks range in strike between 

 N. 45° E. to N. 60° E., and in dip from nearly vertical to nearly 

 horizontal. 



Dikes. — We have already spoken of the granite and serpentine 

 as intrusive, forming in certain cases true beds, and again, show- 

 ing genuine vein structure with numerous branchings. Regard- 

 ing the structural relations of the bluish gray trap, there are as 

 yet some doubts. The geologists of the Second Pennsylvania 

 Survey have spoken of it as forming massive trap intrusions 

 between hornblende gneiss — of which the latter is metamorphic. 



This theory, I am inclined to think, will prove to be a mistaken 

 one. In Delaware, as we have said, the gray acided rock runs by 

 indistinct gradations into true araphibolite schist, the many 

 stages of variations being sometimes witnessed in a single quarry, 

 without the slightest structural distinctions. Seams of the gray 

 trap have been seen running through the black hornblende gneiss, 

 but without the least signs of intrusion. That the bluish gray 

 trap may occupy irregular patches more or less lenticular, is no 

 doubt true, but the latter can in no sense be regarded as forming 

 dikes between a metamorphic schist ; on the other hand, all the 

 rocks of the hornblendic belt, from the most acidic gray trap to 

 the true amphibolite schist, belong to a single series of eruptive 

 rocks, having wide lithological variations. That these variations 

 may be the result of a subsequent paramorphosis taking place in 

 the bluish gray trap is quite possible ; it is therefore to be hoped 

 that present petrographical studies of the hornblendic rocks of 

 the State will throw more light on this important question. 



Contortion of Strata. — The crystalline rocks offer the most 

 complicated and striking examples of contortion, presenting 

 nothing, however, which is not characteristic of all metamorphic 

 areas. Close folding is the form generally seen, the line of 

 bedding being either straight, gently or violently contorted. 

 Abrupt anticlinal and synclinal folding is also common, these 

 complex folds being, however, very irregular and much twisted. 



Age and Stratigraphical Order. — ^^The crystalline rocks may be 

 divided into four groups, which have a fixed stratigraphical 

 relation to each other, namely, the hornblendic, the micaceous, 

 the calcareous and the quartzitic. 



The normal order of arrangement of these strata, and thereby 



