33 1 PHYSIOGIIAPHV. CLASS II. 



to be connected with the regular structure, and deserves 

 the attention of all those who may have occasion to observe 

 the mineral in this point of view*. 



2. The species of uncleavable Quartz is altogether 

 united by HAUY with the preceding one of rhombohedral 

 Quartz. In the Wernerian system the great differences in 

 the appearance of its varieties have been the cause of its 

 distinction into a number of species, of subspecies and 

 kinds, many of which have been adopted by mineralo- 

 gists at large. By far the greatest part of the varieties 

 is contained within the species of Opal, excluding from it 

 the Hyalite^ in small reniform, botryoidal, and sometimes 

 stalactitic shapes, white, and generally of considerable 

 degrees of transparency, and the Menilite^ in tuberose 

 forms, and for the rest possessing almost the opposite pro- 

 perties of Hyalite. Menilite has been subdivided into 

 Irown and grey Menilite, according to colour : some of the 



* There is a specimen in Mr ALLAN'S cabinet actually pre- 

 senting traces of cleavage in one direction, reflecting a most 

 beautiful green colour, which is collected into a faint image, if 

 brought sufficiently near the eye. The colour of the specimen 

 itself is a dark wax-yellow ; it is translucent, and presents by 

 refracted light a nearly hyacinth-red colour. The play of light 

 is interrupted and re-appears, alternating in lines, similar to 

 he effect of regular composition in Labradorite. There is ap- 

 parently one cleavage in each individual ; the cleavages from 

 two individuals meet at angles of about 109. In the cabinet 

 of Mr VON MORGENBESSEII at Vienna, there is a white spe- 

 cimen, of a similar description, but consisting of larger indivi- 

 duals, and without the alternation of particles indicative of re- 

 gular composition. From a great number of observations, Dr 

 BREWSTER concludes that the play of light depends upon 

 openings in the interior of the mass of Opal, which are not fis- 

 sures, but of a uniform shape, aud reflecting the tints of NEW- 

 TON'S scale. In some varieties of Hydrophane they are so 

 large that these colours cannot be any longer reflected by the 

 included air ; but they appear when filled with water, and of 

 still higher tints if filled with fluids possessing a high refractive 

 power. Other varieties, in this case, only become transparent^ 

 but do not shew anv colours. H. 



