448 W. J. Mo Gee — Three Formations of 



ously. The region was probably for a long time the scene of 

 eruptive activity. At different periods different types may 

 have been produced which broke through these already solidi- 

 fied. The quartz-mica-diorite near Montrose Station seems to 

 be a later intrusion into the older and more basic norite. 



This will conclude what the writer has to say on the massive 

 rocks of the " Cortland t Series." These are however so ex- 

 tremely varied that their study can hardly be said to be more 

 than begun. It is earnestly hoped that some one may in future 

 work out all their manifold variations and relationships more 

 completely than the writer, at such a distance from the field, 

 has lfeen able to do. 



Enough perhaps has already been said regarding the nature 

 and mode of occurrence of these rocks to place their truly 

 eruptive nature beyond all question; nevertheless all the evi- 

 dence bearing on this point may be more advantageously sum- 

 marized at the conclusion of the next and final paper, which 

 will deal with the phenomena of contact metamorphism pro- 

 duced by the massive rocks in the adjoining schists. 



Petrographical Laboratory. Johns Hopkins 13 niversity, 

 Baltimore, Jan. 27, 1888. 



Art. XXXIX. — Three Formations of the Middle Atlantic 

 Slope ; by W. J. McGee.* 



(Continued from page 388.) 



Resume. — The Columbia formation consists of a series of 

 subestuarine and submarine deltas and associated littoral de- 

 posits, occupying the entire Coastal plain of the Middle At- 

 lantic slope up to altitudes ranging from about 100 feet in the 

 south to over 400 feet in the north ; the delta phase found at 

 the mouths of the great rivers is bipartite, but the littoral 

 phase overspreading the rest of the area is indivisible ; its ma- 

 terials — which are derived largely from the Potomac formation 

 and other local terranes and partly from the Piedmont and 

 Appalachian regions — increase in coarseness northward, and 

 are (in part) evidently ice-borne ; it reaches greatest volume 

 along the principal waterways and near the present coast ; it is 

 destitute of fossils at high levels and in its lower (and ice- 

 borne) portion, but at lower levels and higher horizons yields 

 remains of marine animals of recent and local species ; it is 

 connected with an extensive series of shore-lines and terraces ; 



* Plates VI and VII are issued with this number. The parenthetical clause 

 in the second line of page 137 of this series should read vjhich he is disposed to 

 refer to the Jurassic. 



