Th BRAZILIAN STONE, 89 
ture, depending upon many circumftances ; and one which it 
‘may be very difficult to inveftigate among the {mall particles of 
this ftone. 
WE are now to endeavour to difcover the peculiar ftru@ure 
or conftitution of this Brazilian ftone, by examining, as mi- 
nutely as we can, the form and fubftance of its conftituent 
parts, and that particular texture by which they are united. 
Upon the upper and under furfaces of this thin ftratum of 
ftone, there is to be perceived a certain ftructure, which has a 
refemblance to a fibrous ftru€ture, but which is truly not fi- 
brous. From more accurate infpection, it appears to arife from 
the reflection of longitudinal fpecular plates, which are all re- 
gularly arranged in one direction, confequently are parallel to 
each other. This gives fome refemblance to a fibrous ftruc- 
ture. 
WueEN I examined thefe places with a microfcope, I could 
perceive nothing like mica in them ; but they feemed to me to 
be the impreffion of mica which had fo formed the tranfparent 
fubftance of the ftone. In examining the tranfverfe feGtion of 
the ftone, there appears nothing heterogeneous in its confti- 
tution, nothing micaceous, nor any diftinét mark of ftratifica- 
tion. The ftone is porous or fpungy, feemingly compofed of 
nothing but pure tranfparent quartz, and fhows neither a fi- 
brous nor a laminated ftru@ture. It refembles nothing fo much 
as a comprefled ftratum of fnow. I now almoft gave up every 
fufpicion of mica in the compofition of this ftone; and this 
» will ferve to fhow how deceitful may be certain appearances. 
My next operation was to fplit this ftone in the direction of 
its ftratification, by preffing in the point of a knife. Here I 
found that this ftratified body has truly a foliated ftru@ture, and 
a certain tenacity in the direction of its ftratification, which 
admits of flexure before it breaks. The fame ftriated appear- 
ance is here to be perceived in the internal horizontal feCtion, 
as was obferved upon the upper and under furfaces of this ftra-° 
Vou. IIL M tum; 
