123 



in the formation, and which appear within three miles of the gneiss 

 at Trenton. A large portion of the pebbles, which frequently are 

 half an inch in diameter, consist of quartz and felspar; the latter 

 occasionally in a decomposing state. With these are mingled 

 flatter pieces of red shale, the whole being bound together by a 

 small amount of ferruginous matter acting as the cement. Much 

 of this latter rock is merely a coarse sandstone, and in the vici- 

 nity of Centrebridge, and at intervals for a mile and more above, it 

 occurs of a quality excellently fitted for architectural uses, having 

 been fully tried in the bridges upon the upper end of the Feeder. 



This rather arenaceous part of the formation, extends nearly 

 to the mouth of the Lockatong, if we follow the bend of the river. 

 Commencing a little above that stream, we meet with an elevated 

 table-land, the surface of which is about four hundred feet above 

 the Delaware. At the valley of the river this is about three miles 

 wide, but it expands towards the northeast, until it reaches the 

 valley of the South Branch of the Raritan. Its low^er edge is traced 

 by a line, commencing at Bull's Island, and passing west of Ser- 

 geantsville and Flemington. Here it bends to the north to follow 

 the South Branch to within three miles of Clinton. The upper or 

 northwestern limit is less clearly defined, as the highly indurated 

 strata pass, by nearly insensible shades in some places, into the 

 less altered rocks of llie tract to the northwest. An approxima- 

 tion, however, to its boundary, will be had by drawing a line 

 from a little below vSmithsville, on the Delaware, through Pitts- 

 town, to near the south branch of the Raritan. 



Throughout this area, the rock preserves a moderately uniform 

 external character; being a highly indurated altered shale and 

 sandstone, the prevailing colour of which is a very dark dull blue, 

 sometimes a deep gray, and sometimes an olive-green. It has a 

 great tendency to split into rhomboidal fragments, with a some- 

 what splintery fracture ; and certain varieties yield, when struck, 

 a clear ringing sound, which has procured it the name of clink- 

 stone in the neighbourhood.* 



The tract where this rock exists, goes in Hunterdon county 

 under the name of the Swamp, owing to the wet character of the 

 prevailing soil. Little or no true trap rock is visible in the dis- 



* True clinkstone, or phonolite, is a felspar rock of the trap family, usually 

 fissile, and is sonorous when struck ; whence its name. 



