24 NEW YORK STATE MUSEUM 



tween Piermont and the state line the broad depression whose ele- 

 vation is about 200 feet. The accord in the hight of these gaps is 

 probably not a mere coincidence but the result of certain geo- 

 graphic conditions which no longer exist. The gaps are undoubt- 

 edly the result of erosion and probably mark a period when the 

 elevation of the land was about 200 feet less than at present. At 

 this time the streams cut down their channels across the trap 

 ridge about to base level. Subsequent elevation of the region 

 seems to have given opportunity for readjustments in the drain- 

 age lines, by which most of the streams escaped from the hard 

 trap rock, and gained courses on the softer sandstones. ,A stream^ 

 however, maintained its course across the sag near Sparkill, and 

 has since eroded a narrow gorge through the ridge almost to the- 

 present sea level. 



Relation to the sandstones. The trap forms an intrusive sheet 

 or sill lying between the layers of sandstone and dipping with 

 them at a gentle angle westward. This relationship accounts for 

 the steep and precipitous eastern face and the gentler westward 

 slope. The trap does not, however, everywhere occupy the same 

 horizon between the sandstones. There are many places, partic- 

 ularly in New Jersey, but also in New York, where it visibly 

 crosses the sandstone beds from one horizon to another. These 

 unconformable contacts have been found along both the under 

 and the upper margins, and afford clear and indisputable evidence 

 that the trap was forced into its present position between the 

 sedimentary rocks in a molten condition, and that it did not come 

 to the suface and overflow as do lava sheets from volcanos. 



For most of the way from Bergen Point (N. J.) to Rockland 

 lake the trap is intercalated between the beds of sandstone at 

 slightly discordant horizons and dips westward with them. There 

 is, however, evidence at a number of localities along the western 

 side to suggest that the steeply inclined vertical fissure up which 



the molten rock ascended is located close along this margin. 1 



~-^ — — — ____^ — > 



iThis relationship was noted many years ago by N. H. Darton, U. S. 

 geol. sur. Bui. 67, p. 37. So also were a number of the local details to 

 which I shall make reference below. In the preparation, however, of this 

 report I have made use only of my own observations in the field, unless 

 otherwise expressly stated. 



