THEORY of the EARTH. 249 



ibmetimes a third cryftallization, fuperincumbent on the fe- 

 cond, in like manner as the fecond was on the firft. I {hall 

 mention fome particulars. 



I HAVE one fpecimen, in which the primary cryftals are fili- 

 ceous, the fecondary thin foliaceous cryftals of deep red but 

 tranfparent iron-ore, forming elegant figures, that have the form 

 of rofes. The tertiary cryftallization is a frofting of fmall fili- 

 ceous cryftals upon the edges of the foliaceous cryftals. 



IN other fpecimens, there is firft a lining of colourlefs filice- 

 ous cryftals, then another lining of amethyftine cryftals, and 

 fometimes within that, fuliginous cryftals. Upon thefe fuligi- 

 nous and amethyftine cryftals are many fphericles or hemi- 

 fphe'res of red compact iron-ore, like haematites. 



IN others, again, the primary cryftals are filiceous, and the 

 fecondary calcareous. Of this kind, I have one which has, up- 

 on the calcareous cryftals, beautiful tranfparent filiceous cryftals, 

 and iron-fphericles upon thefe. 



Lajily, I HAVE an agate formed of various red and white coats, 

 and beautifully figured. The cavity within the coated part of 

 the pebble is filled up without vacuity, firft, with colourlefs 

 filiceous cryftals ; fecondly, with fuliginous cryftals ; and, laftly, 

 with white or colourlefs calcareous fpar. But between the fpar 

 and cryftals there are many fphericles, feemingly of iron, half 

 funk into each of thefe two different fubftances. 



FROM thefe facts, I may now be allowed to draw the follow- 

 ing conclufions : 



F'trfi ', THAT concretion had proceeded from the furface of 

 the agate body inwards. This necefTarily follows from the na- 

 ture of thofe figured bodies, the figures of the external coats 

 always determining the fhape of thofe within, and never, con- 

 trarily, thofe within affecting thofe without. 



'idly, THAT when the agate was formed, the cavity then con- 

 tained every thing which now is found within it, and nothing 

 more. 



I i 



