218 Dr Brewster's Description of WitJwmitc, 



Art. VIII. — Description qfWithamite, a new mineral species 

 Jbund in Glenco. By David Brewster, LL. D. F. R. S. 

 Lond., and Sec. R. S. Ed. 



The mineral, of which I propose to give a short description, 

 was found by Henry Witham, Esq. in Glenco, in Argyleshire, 

 during a mineralogical excursion which he made to the High- 

 lands of Scotland, in the month of August 1824*. 



The mineral occurred in a trap-rock of a reddish brown co- 

 lour, and was disseminated in grains, or in small masses, which 

 shot out into regular crystals in the larger cavities. These 

 crystals are very minute, seldom exceeding the lOOdth part of 

 an inch in diameter. They occur in radiated spherical groupes, 

 the central parts of which are of a light red colour, while, to- 

 wards their circumference, they terminate in separate crystals,, 

 which, by reflected light, have a dark red colour, like that of 

 arterial blood. Very fine groupes of transparent crystals 

 sometimes penetrate the quartz which occasionally accompa- 

 nies the mineral ; and when thin chips of this quartz are im- 

 mersed in Canada balsam, which has nearly the same refrac- 

 tive power as quartz, and submitted to a powerful microscope, 

 the separate crystals of the mineral are displayed with peculiar 

 advantage. 



Having succeeded in detaching some minute crystals from 

 specimens submitted to me by Mr Somerville, who accompanied 

 Mr Witham to the Highlands, I found that they had generally 

 the form of an irregular six-sided prism with flat summits, and 

 that in the broken crystals there was an imperfect, though to- 

 lerably distinct, cleavage, perpendicular to the axis. In a fine . 

 specimen belonging to Mr Witham, I have since observed 

 various faces upon the summit of particular crystals, but they 

 are too minute to admit of measurement. The following are 

 the angles of the prism, which I obtained by the reflecting go- 

 niometer. See Plate VIII. Fig. 1,2. 



A upon B 128° 20' D upon E 166° 30' 



B C 63 20 E F 76 



C — — D 16» 20 F— A 118 30 



