Biaxial character of Pyramidal and Rhombohedral Crystals. 191 



any that is as much weathered as this Euba quartz now and then 

 is. It may be assumed for a fact that it is more liable to become 

 so than is its neighbour the paradoxite. 



My friend M. Reich, the Superior Councillor of Mines, had 

 the kindness to examine this quartz chemically for me, and 

 found therein nothing but silica, with traces of oxide of iron 

 amounting to \ per cent. Professor Wohler has been so good 

 as to say he will, prepare some silicon from it and submit it to 

 a further examination. 



This quartz occurs at Euba in veins, which are four in number 

 in all. Three of them do not exceed 1 inch or 2 inches in width, 

 but the fourth is above 2 feet thick. In the former, quartz and 

 paradoxite alone occur ; but in the latter, fragments of porphyry 

 are met with, as also fluor in considerable quantity, and some 

 calcite and mica. 



As I was familiar with paradoxite elsewhere only in connexion 

 with tin- veins, and as the blue fluor, as it occurs here, is specially 

 associated with veins of that kind, I caused large fragments of 

 the general mass of the veins to be broken up, stamped, and 

 washed, when, lo, tin-ore was obtained by washing, from which 

 beautiful metallic tin was got before the blowpipe with oxalate 

 of potash upon charcoal. The tin-vein formation, therefore, 

 which hitherto has been held to be one of the oldest, is a very 

 recent one, for the veins in question occur in the lower new red 

 sandstone. 



There are also other varieties that may be held to belong to 

 this quartz, especially, for instance, the so-called star-quartz from 

 the vicinity of Bautzen in Saxony, and from the neighbourhood 

 of Hohenelbe in Bohemia. These are of the hardness of adu- 

 laria. They consist of wedge-shaped, bacillary, aggregated pieces 

 radiating in a stellar form; they are, however, quite cloudy, 

 and for the most part somewhat weathered. The Euba quartz 

 assumes partially the same structure ; but it is rarely stellar, it 

 is commonly fascicular. I am acquainted also with another 

 star-quartz from the Hill of Molignon in the Tyrol. 



As I have for the last four years occupied myself to my utmost 

 with optical studies, I will here mention, quite in a preliminary 

 way, and as concisely as I can, some of the principal results at 

 which I have arrived. 



The grossular garnet from Siberia is in one tetragonal axis uni- 

 axial, as is the case with essonite and almandine. The garnets 

 whose specific gravity is the highest, namely the manganese gar- 

 nets, are optically isotropic. 



In the pyramidal system, I have found that all transparent 



