50 On Quartz. By Mr. R. J. Moss, F.CS. 
several laminze varying in thickness, the thinner laminze some- 
times passing into thicker layers; there are some small spots 
on the faces of the crystals, where the laminated structure entirely 
disappears, thus producing the patches of vitreous lustre already 
referred to. A crystal immersed in aqueous hydrofluoric acid soon 
loses its peculiar lustre, the coating being irregularly dissolved, 
the faces of the crystals are left rough and uneven. In some 
places the coating exhibits numerous cracks, which intersect 
irregularly and correspond more with a vitreous than with a 
crystalline structure. As the crystals are simply capped with 
laminz of quartz, I concluded that this “ Pearl Quartz,” as the 
Rev. Mr. Reade proposes to call it, must be regarded as a variety 
of cap-quartz, the lustre being entirely due to the great reflection 
of light from the surface of the laminz of which the caps are 
composed, The origin of this peculiar structure cannot, of course, 
be determined with certainty ; it is, however, obvious that the 
growth of the crystal has not proceeded under conditions of 
uniform regularity, on the contrary it would appear that occasional 
changes in the conditions of growth have led to the deposition 
of thin films of some foreign substance on the faces of the crystals. 
The succeeding growth of quartz has either been separated in 
this way from the former growth by a thin layer that has sub- 
sequently disappeared, or else it has been pseudomorphous after 
the coating. Considering that the quartz was associated with 
calcite, and also the frequency with which this mineral is found 
as an incrustation on quartz, it is not improbable that the 
substitution of quartz for calcite is the true origin of the laminated 
structure. There are crystals of siderite embedded in some of 
the masses of quartz. This mineral also is frequently replaced by 
quartz. 
