A GEOLOGICAL HISTORY. 47 



make different modifications ; there are natural 

 chemical changes always going on, as a proof of 

 these changes, dip a brick in a solution of Muriate 

 of Soda, (brine of common salt,) put some mason's 

 mortar on each side, lay it between two other 

 bricks and keep it in a damp place about a fort- 

 night ; a salt will crystalize at the sides of the one 

 which had been diped in brine, but on the upper 

 and under ones no crystalization will take place, 

 these crystals will not be (Muriate of Soda) com- 

 mon salt, but will be changed to Carbonate of Soda 

 an alkaline Carbonate. This process is going on, 

 in all our brick walls, in new buildings,* where 

 the bricks in transporting have been left at the 

 landings and soaked by the tide-water, or where 

 they have been exposed on the deck of the vessel 

 to the salt spray. 



4th Greenstone Slate. — (See plate 2, Jig. 4.) 

 This is an horizontal layer, between the Sand- 

 stone and the vertical trap, of which last it is a 

 member. It is lammellar, and between its laminae 

 it has in many places, minute crystals of Datholite, 

 and all the minerals of this formation. It is seen 

 most beautifully at the shore under the bluff at 

 Weehawken for more than half a mile, with its 



* See Dr. J. C. Jay's Walls to Cabinet, 22 Bond Street. 



