42 



three and a half miles broad, narrowing in its course to the north- 

 northeast, until, by the overlapping of the newer beds upon it, 

 the visible portion of the formation fines away almost to a point, 

 about the sixth mile-post upon the Delaware and Raritan canal. 

 The triangular area which it forms has the valley of the Assun- 

 pink very nearly for its southeastern boundary, while its north- 

 western margin is formed by the lower members of the overlying 

 middle secondary sandstones. Throughout the whole included 

 space, the mineralogical character of the rock is extremely well 

 marked. It is usually a triple mixture of quartz, felspar, and 

 hornblende, the latter being frequently replaced by mica. Like 

 the rock of the other primary zone of the State, the Highands, 

 it goes very frequently under an improper name, being called a 

 granite, and sometimes a sienite. Its well-marked dip and strati- 

 fication, its occasionally schistose structure, and the decisive fact 

 of its running in strict continuity with the acknowledged gneiss 

 rock of the Schuylkill above Philadelphia, are sufficient to esta- 

 blish its claim to be considered a portion of the great Atlantic 

 belt of gneiss. 



A little north of Trenton, and near its iDorder, there is a 

 quartzose variety of the rock, containing a little mica, giving it 

 the laminated form, but the mass of the rock is a close-grained 

 stratified mixture of felspar and quartz. This band consists of an 

 intimate mixture of quartzose and felspathic matter fused together. 

 It seems to be continuous with the felspathic rock of Barrel Hill, 

 in Montgomery county, Pennsylvania. Splitting into rather well- 

 formed large slates, and having a smooth surface, it furnishes a 

 very good flag stone for the walks and steps in Trenton. 



Wherever the stratification of the gneiss can be seen, it is found 

 to dip at a steep angle, nearly 70°, to the southeast. And there 

 can be no doubt that it underlies unconformably both the upper 

 secondary, or greensand deposits on its southeast, and the middle 

 secondary, or argillaceous red sandstone formation, on its north- 

 west. One variety of the rock possesses the general aspect of a 

 sienite, and another often contains such an excess of hornblende 

 as to cause it to resemble closely a greenstone or basalt, for which 

 it might be taken were it not for the stratified structure evident 

 in almost every mass. In some portions the quartz is blue, semi- 

 transparent, and opalescent, and the hornblende and felspar show 



