1892.] NEW YORK ACADEMY OF SCIENCES. Tl 



although to ordinary observation the rocks seem harder. A ledge 

 was found that could not have been more than 15 3^ards from the 

 syenite, but the slide shows no appreciable mineralogical change. 

 The sandstone is, however, rendered so crystalline at times, that 

 it is difficult to detect its clastic character without a thin section. 



Connection beticeen present topography and crystalline struc- 

 ture. — Professor W. M. Davis has been giving much attention of 

 late to the topographic development of this region.^ and the approxi- 

 mate geological dates of many of the upheavals and drainage systems 

 have been pointed out. The conclusions have a certain bearing on 

 the age of the syenite. The great dike exhibits over its outcrop a 

 coarsely crystalline structure except in the case of the porphyry, which 

 is probabh" a subordinate narrow intrusion. The orthoclase crystals 

 even reach verv large size, and the granitoid character of the rock 

 indicates that it crystallized at a considerable depth, and was in no 

 sense a surface flow. The outcrop stands now at the level of what 

 Davis calls the Schooley peneplain, which marks the base-level of a 

 post-Triassic system of drainage. The dike must have suffered the 

 erosion of this time, and perhaps of earlier cycles in order to expose 

 its coarsely crystalline portions, and it is reasonable to place its in- 

 trusion at an earlier period. But as regards its age we are only 

 able to say, that it is later than the Oneida conglomerate at the 

 beginning of the Upper Silurian, and before the Triassic or late 

 Triassic. During this long interval, there occurred the Carboniferous 

 subsidence and post-Permian upheaval, and the dike may have been 

 a concomitant occurrence with one of these. 



Geological Laboratory, Columbia College. 



At the close of the paper many specimens illustrating the subject 

 were exhibited, and the discussion was carried on by Prof. Britton 

 and others. 



The Secretary announced that the 50th Anniversary of the Hun- 

 garian Society of Natural Sciences would be commemorated on 

 January 11, 1892. 



Also that members of the Academy were invited to attend the 

 meeting of the New York Section of the American Branch of the 

 Society of Psychical Research to be held at Columbia College, 

 February 10th, 1892. 



Meeting adjourned. 



1 W. M. Davis and J. W. Wood, Jr., Geographic Development of Northern 

 New Jersey, Proc. Bost. Soc. Nat. Hist., vol. xxiv, 1889, 385, 411. W. M. 

 Davis, The Geological Dates of Origin of certain Topographic Forms on the 

 Atlantic Slope of the U. S., G. S. A., vol. ii, 559. 



