20 GEOLOGICAL MEMOIRS. 



The specific gravity of the natural basalt used in the experiment 

 was 2'887. After Uxiviation of the soluble alkaline salt with water, 

 and after drying in the air, until there was no further loss of weight, 

 the specific gravity of the altered basalt was taken ; that of the nu- 

 cleus was 2' 1588 ; that of the less hard external portion was 2*04 23. 



It is evident that in this case the formation of a hydrate had taken 

 place : and this behaviour of the basalt under the influence of water 

 and of the atmospheP€ must prove to be of some interest to geologists. 



[T. R. J,] 



On JuNKERiTE. By A. Kenngott. 



[Min. Notizen, xiv. p. 13 ; and Leonhard u. Bronn's N. Jahrb. f. Min. &c. 1856, 



p. 49.] 



The examination of a specimen of junkerite from Poullaouen, in 

 Brittany, confirmed Breithaupt's statement that junkerite is rhom- 

 bohedral and belongs to siderite. It occurs as crystals on quartz, 

 which form a crystalline coating, and are also met with as single 

 crystals. The isolated crystals, although very small, can be recog- 

 nized as exhibiting a combination of an acute rhombohedron, in the 

 opposition wzR' (taken on the primary form R of siderite), with the 

 basal planes. The rhombohedral planes are somewhat lustrous and 

 convex ; the basal planes, rough and dim. The convexity arises 

 from the presence of a scalenohedron, which sharpens the lateral 

 angles of the rhombohedron, the planes of which however by the 

 convexity of the rhombohedral planes can form no distinct angles of 

 combination. The crystals, completely cleavable parallel to the 

 planes of the primary form R (which is the case also in siderite), are 

 yellowish-brown and transparent.. [T. R. J.] 



On remarkable Crystals q/" Quartz and Fluor-spar. 

 By A. Kenngott. 



[Min. Notizen, xiv. p. 20,22 ; an'd Leohn. u. Bronn's N. Jahrb. f. Min. &c. 1856, 



p. 39.] 



In the eclogite of the Sau-alps in Carinthia remarkably peculiar 

 crystals occur, which are there known by the name of ** white topaz." 

 Close examination, however, shows that these are either imperfect or 

 misformed crystals of quartz. 



At Schlackenwald in Bohemia the joint-surfaces of a fine-grained 

 granite are covered over with small quartz-crystals, and on this coat- 

 ing are crystals of fluor-spar of two different kinds, occurring side 

 by side, viz. green octahedrons and violet-blue rhombic dodeca- 

 hedrons. The latter, fully formed, appear at first sight as triakis- 

 octahedrons, on account of white stripes in the position of the longer 

 diagonals. On close examination, it is seen that the layers corre- 

 sponding to the three chief sections are colourless, whilst the rest of 

 the mass of the crystals is violet-blue. [T. R. J.] 



