41 



taining it is washed in a stream of water, running through 

 troughs ; after which it is sifted, and the increase in the product 

 proves that the extra labour spent in preparing it is economically 

 bestowed. 



In two or three places, in the iron region, magnetic separating 

 machines are used to clear the ore of the foreign mineral matter 

 with which it there is mixed. They are applied, of course, only 

 when the quantity of non-metallic matter is so large as to consti- 

 tute a motive for removing it; nor are they admissible except 

 where the ore is highly magnetic. One has been used at Ring- 

 wood, and another at Hibernia. 



It is a curious circumstance, that the igneous injections, pene- 

 trating the older secondary limestone of the chain of valleys, 

 north of the main range of the Highlands, from Amity, in New 

 York, to Scott's Mountain, producing a striking change in its 

 structure, though sometimes wholly metalliferous in their nature, 

 rarely embrace veins of magnetic iron ore such as belong to the 

 .gneiss of the Highlands. 



Some of the veins of magnetic iron ore, penetrating the gneiss, 

 may, therefore, claim an earlier date than those metalliferous 

 and non-metalliferous veins which traverse only the subsequently 

 formed blue limestone. 



The highly singular phenomena, connected with the intrusion 

 of these last named veins in the limestone rocks of the valleys, 

 will demand a particular description, which it is deemed proper, 

 however, to defer, until the general geology of the older secondary 

 strata shall have been previously discussed. 



Gneiss Formation of Trenton. — The very extensive belt of 

 stratified primary rocks, which ranges nearly parallel with the 

 Atlantic coast, and forms the western limit of the tide in the 

 rivers of Virginia, Maryland, Delaware, and Pennsylvania, after 

 gradually contracting in width, crosses the Delaware at Trenton, 

 and soon entirely vanishes, dwindling to a point about six miles 

 to the northeast of the state metropolis. Its further course 

 through the State is concealed by the overlying horizontal depo- 

 sits of clay and sand, referable to the greensand series. The 

 formation again comes into view in Staten Island, Long Island, 

 and New York. 



Where it crosses the Delaware, this belt of gneiss is about 



4* 



