no MINERALOGY AND LITHOLOGY. 



Magnesia, 2.19 1.64 



Ignition, ,27 .68 



101.81 99.81 



This very great difference in the two analyses is explained by the cir- 

 cumstance that the staurolite with a high percentage of silica is rendered 

 impure by the enclosure of quartz, as has been proved by a number of 

 investigators ; and these irregular varieties, when purified from the quartz 

 by treatment with hydrofluoric acid, as was done by Rammelsberg, give, 

 on analysis, a composition which corresponds also with the formula. 

 This admixture of quartz is apparent in the specific gravity which sinks 

 from 3.76 in the purer variety from Franconia to 3.41 in the impure 

 variety from Lisbon. Sections of the Lisbon staurolite show the quartz, 

 which is present as large clear grains scattered through the interior of 

 the crystal. 



Staurolite forms macles, or tessellated crystals like andalusite ; but this 

 is a rare occurrence, and I am not aware of such having been observed 

 save those noticed by Jackson at Charlestown, in our state. These ma- 

 cles of staurolite are made in the same manner as the andalusite macles 

 by the symmetrical arrangement of pure and impure material. Fig. 8 on 

 PI. 2 represents the base of one of these crystals. They are found in 

 the mica slate, which Jackson states gradually passes into an argillite, 

 and with it the character of the crystals changes till they become anda- 

 lusite macles in the argillite. 



This staurolite is not in twin crystals; a rare occurrence, since stau- 

 rolites which are apparently simple usually prove to be compound when 

 cut and examined. These peculiar macles have been examined by Peters 

 and Rosenbusch, and by Jackson. Fig. 8 is taken from Jackson's article. 

 Fig. 8 a is from Rosenbusch' s Mikroscopische PJiysiograpJiie, by which it 

 is shown that the made is not produced by twinning, since the cleavage 

 lines are undisturbed, but that the case is one where a core is surrounded 

 by another crystal, giving to the whole a laminated structure, and the 

 made results from the regular arrangement of impurities and cavities, 

 which are most abundant between the outer and the inner crystal. 



The microscopic characters of minute crystals are usually the same 

 as those of the large ones. In them more or less quartz is found, and 

 apparently simple crystals, with polarized light, are seen to be twins; 



