190 Dr. Clarke's Account of [March, 



are deposited in a dark porous toadstone rock, very like what is 

 commonly called lava at Naples, and which, like the black 

 arenaceous particles found with the crystals of pyroxene, is 

 easily fusible before the common blow-pipe, into ajef-black shin- 

 ing glass, externally resembling black sealing-wax. The second 

 specimen has the appearance of a substance, acted upon by fire ; 

 it has a reddish brown appearance, looking like a cinder ; being 

 full of innumerable pores, and having the spongy aspect of the 

 scoria ejected from Vesuvius. Besides containing these splen- 

 dent crystals, it has also a few imbedded crystals of more opaque 

 pyroxene ; it fuses, like the preceding specimen, into a je^-black 

 glass. The third specimen is a light grey, friable, earthy, aggre- 

 gate, in which the crystals of pyroxene lie imbedded with dark 

 roundish granular pieces of basalt or trap in a crumbling mass, 

 that easily separates between the fingers. This last fuses, like 

 the two former varieties, into a jet-black glass. 



I have now done with the description of the minerals from 

 Jean Mayen island ; and shall proceed with an account of an 

 interesting mineral from Sweden of a very different nature. This 

 substance was sent to Dr. Ingle of this University, from Mr. 

 Swedenstiema of Stockholm, under the name ofpetalite. Find- 

 ing it to be altogether new to me, and that its nature was not 

 likely to be made known by the name which had been given to 

 it (no account of its analysis having appeared) ; I undertook to 

 examine it chemically ; requesting Mr. Holme to do the same ; 

 that by comparing the result of our observations we might be 

 able to state the constituents with all the accuracy in our power. 

 It will be proper, in the first place, to describe the mineral, and 

 to point out those characters which induced me to consider it as 

 anew substance. 



Externally it resembles common xohite quartz, and to such a 

 degree, that a very eminent mineralogist to whom Dr. Ingle 

 exhibited this mineral, considered the specimen shown to him as 

 a piece of quartz. Upon a nearer examination, however, it will 

 be found to differ in fracture from quartz. It admits of a 

 two-fold cleavage parallel to the sides of a rhomboidal prism : 

 two of which parallel to each other are splendent, and the other 

 two are dull. Hence there is an evident appearance of crystalli- 

 zation ; but I was unable, owing to the want of two contiguous 

 reflecting surfaces, to ascertain the angle of their inclination in a 

 satisfactory manner. Its specific gravity, nearly that of quartz, 

 equals 2*45. Its colour is white, but an almost imperceptible 

 pinkish hue may be discerned when it is attentively examined. 

 It is hard enough to scratch glass; although it may be rased 

 by a knife. Exposed to the common blow-pipe it is almost 

 infusible; but after an intense heat has been for some time con- 

 tinued, it exhibits a glazed superficies, which examined by a 

 lens appears full of minute bubbles. When triturated and reduced 

 to an impalpable powder in a porcelain mortar, it has the white- 



