C 2 59 ] 
that where Cockle is plenty, Spar-Nodules round as 
Musket Balls, and black, in Sockets of the fame 
Colour and Subftance, are frequently found. But as 
this Pebble was not black, as Cockle always is j it 
may therefore be queftioned, whether the Shape of 
it may not be owing to fome metallic {viz. Iron or 
Copper) Principle, rather than to Cockle, and whe- 
ther Cockle itfelf be not more probably indebted to 
other Powers for its orbicular Nodules, in fuch 
Sheaths, than derive them from any inherent Acti- 
vity of its own. — The Exterior of the Shell or Socket 
has a thin Incruftation of gritty cinereous Mundic. — 
It came out of JVheal Royal Mine, in the Parifh of 
Cambron , Cornwall. 
Of Incntftations. 
The next Appearance of Spar Bodies, which I 
(hall here take Notice of, is that of Incruftations ^ 
thefc fometimes make one continued Sheath, Lump, 
or Mafs, and inclofed in them we find Cormjb 
' Diamonds , Grains of Tin, and other adventitious 
Bodies, plainly of a different Texture and Colour 
from the Crufts which furround them 5 fo that In- 
cruftations muft be cautioufly diftinguiftied from the 
entire Sheaths, or Lamina;, which compofe the Co- 
lumn of hexagonal Cryftals, and which are really 
form'd at the fame time with that Column (13); where- 
as Incruftations are additional, and after Concretions 
made on the before fettled originalGrains andColumns, 
(14) fometimes thefe Incruftations, are but Sprink- 
(12) Cockle is a black, fliining, light Stone, free of all Metal, 
different from Mock-Lead, common in the Tin Mines of Cornwall. 
(13) See p. 274. Note 2 6 . 
