356 Mr. T. S. Hunt's Examinations of some Felspathic Rocks. 



felspar, which are sometimes from one-tenth to one-half an inch 

 in diameter, but often take the form of large imperfect crystals, 

 frequently 12 inches long, and 4 or 5 inches wide. These 

 dimensions correspond to the faces M and T, while the face P, 

 characterized by its perfect cleavage, is from half an inch to 2 

 inches broad. Twin crystals sometimes occur having a compo- 

 sition parallel to M. 



Hypersthene is met with throughout the rock in flattened 

 masses, which although variable and irregular in their distribu- 

 tion, exhibit a general parallelism ; they are occasionally 4 or 

 5 inches in length and breadth, by an inch or more in thick- 

 ness, and are separated from the granular felspathic rock by a 

 thin film of brownish-black mica. Titaniferous iron ore is also 

 found in grains and lenticular masses, occasionally an inch or 

 two in thickness ; these occur in the granular base, and gene- 

 rally near the hypersthene, but grains of the mineral are occa- 

 sionally found in the crystalline felspar. Quartz in small grains 

 is imbedded in the titaniferous iron, but was not observed else- 

 where in the rock, nor have any other minerals than these been 

 detected. In the specimens of the rock which I selected on the 

 spot for examination, the crystalline felspar constitutes from one- 

 half to seven-eighths, while the hypersthene does not equal more 

 than T §jjdths, and the titaniferous iron more than T — dth of the 

 whole; the amounts of quartz and mica are insignificant. In 

 other portions of the rock, however, the proportion of the ore 

 may equal 5 per cent., and in some parts the amount of hyper- 

 sthene is nearly as great. By the action of the elements, the 

 surface of the rock becomes of a dull opake white ; the cleavable 

 masses of felspar are, however, less affected than the granular 

 portion, and by their obscure reddish colour are distinctly visible 

 on the weathered surfaces ; this change extends but a very little 

 distance into the rock. The iron ore of course remains unaltered, 

 but the dark brown hypersthene becomes lighter, and inclines to 

 a pinchbeck-brown. 



The felspar is triclinic in cleavage ; the angle of P : M = about 

 80° 80'. Cleavage with P perfect, with the other planes distinct; 

 P is often delicately striated, and sometimes curved. Hardness 

 = 6, and density 2 - 667 to 2*674. Lustre vitreous, sometimes 

 pearly on P; colour flesh-red, passing into reddish, greenish 

 and grayish-brown ; the surfaces sometimes mottled, but the 

 red always predominating. 



The following analyses were made of three different specimens, 

 which were carefully selected, and after being pulverized, dried 

 at 212° F. The earthy ingredients were determined after fusion 

 with carbonate of soda, and the alkalies by the method of Dr. J. 

 L. Smith, which consists in igniting for some time the finely 



