340 DESCRIPTIVE MINERALOGY. 



The feldspar is white and red, and often affords large cleavages. It is sometimes beautifully 

 variegated with narrow veins and small masses of quartz. 



It is scarcely necessary to add, that common feldspar is found in various other parts of this 

 county. 



Washington County. Feldspar is found in the town of Putnam, about a mile south of 

 Black point on Lake George, where it is associated with pyroxene and mica. 



Westchester County. Feldspar and hornblende occur at Peekskill ; and at Tarrytown 

 there is a greenish grey variety, in extensive beds from three to nine feet thick, connected with 

 mica slate. 



The same mineral, often giving large and good cleavages, is often found in the vicinity of 

 West-Farms, New-Rochelle, and along the line of the Croton aqueduct near Yonkers and 

 elsewhere. It is sometimes of a reddish tint, but I am not aware that perfect crystals have 

 been obtained. 



APPENDIX. 



Porcelain clay, or Porcelain earth. The localities of this valuable material hitherto observed 

 in this State, are enumerated in the first part of this work (page 59). It is chiefly found in 

 the counties of Essex and Warren, where it is thought to be abundant. This is supposed 

 to be similar to the kaolin of the Chinese, from which the finest specimens of porcelain are 

 manufactured. Should it be found in sufiicient abundance, it must become one of the most 

 important of our mineral products. 



It has already been stated that the occurrence of this clay is generally ascribed to chemical 

 changes effected in various kinds of felspathic rocks, although the precise nature of these 

 changes does not seem yet to be understood. 



This clay is principally composed of silica and alumina, and is considered a silicate of alu- 

 mina, although small quantities of potassa and magnesia are also occasionally found with these 

 ingredients. 



It appears that the talcose granite, or the variety called protogine, is that which most fre- 

 quently furnishes beds of porcelain clay ; and Mr. Boase has suggested that the presence of 

 magnesia in the feldspar of this rock may, among other causes, contribute towards the ex- 

 traordinary change which this rock experiences. "Thus, the magnesia" which it contains 

 " may absorb carbonic acid, as well as the alkali from the percolating water ; and so great is 

 its tendency to combine with two proportions of this acid, that even one part of the carbonate 

 will attract the acid of the other, so as to pass into the bicarbonate of magnesia ; in which 

 state being soluble in water, it would be speedily removed." This he thinks would in some 

 measure explain the origin of kaolin, and also account for the small quantity of the earth re- 

 maining in the porcelain clay. Indeed, he states that in some samples which he has examined, 

 not a trace of magnesia could be detected.* 



* H. S. Boase. London and Edinburgh Philosophical Magazine and Joumai of Science. 3d series. X. 348. 



