The BRAZILIAN STONE. 89 



ture, depending upon many circumftances ; and one which it 

 may be very difficult to inveftigate among the fmall particles of 

 this ftone. 



We are now to endeavour to difcover the peculiar ftruclure 

 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 ftruclure, which has a 

 refemblance to a fibrous ftruclure, but which is truly not fi- 

 brous. From more accurate inflection, 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. 



When I examined thefe places with a micro fcope, 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 fection of 

 the ftone, there appears nothing heterogeneous in its confti- 

 tution, nothing micaceous, nor any diftincl 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 ftruclure. It refembles nothing fo much 

 as a compreffed 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 prefling in the point of a knife. Here I 

 found that this ftratified body has truly a foliated ftruclure, 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- ' 



"Vol. III. M turn ; 



