ON THE DIAJIOND, OPAL, &C. 
557 
in various primitive rocks. But its principal distribution 
is in rocks of the secondary class, particularly in traps and 
porphyries. In these it is arranged in veins, drusy cavities, 
and imbedded masses, and assumes the various forms of 
precious opal, common opal, semi-opal, wood-opal, and 
menilite. The menilite and wood-opal are the most mo- 
dern of these, — the first occurring imbedded in the adhe- 
sive slate of the Paris formation, the other in tuffaceous 
rocks, of the nature of trachyte. The opals are found 
sometimes so soft,, that they can be flattened between 
the fingers. The alluvial rocks are not without opal, 
for it is daily forming by deposition, from the waters 
of various springs, particularly hot springs, as those of Ice- 
land. From the magnitude and abundance of these 
springs, in many regions of the earth, and the quantity of 
siliceous matter they deposite, we can form a general esti- 
mate of the great quantity of opaline matter formed in this 
way. We have now traced opal, from the primitive to the 
newest rocks, thus proving that it is one of those minerals 
which have an extensive geognostical range, and which are 
still forming in the mineral kingdom ; but one of the most in- 
teresting features in its natural history remains to be noticed. 
I allude to its formation by the organic powers of plants. 
It is well known to botanists, that silica occurs in conside- 
rable abundance in several tribes of plants, and that it 
communicates to the parts of the plants containing it, a 
considerable degree of hardness. The Bamboo is one of 
the most remarkable in this respect, as the earth it eon- 
tains occurs not only in the vegetable structure itself, but is 
secreted from it, and appears in the joints of the plant, in 
solid masses, named Tabasheer, and which bear a strong 
resemblance to opal. We have thus shewn that opal is a 
formation of primitive, secondary, and alluvial strata ; and 
finally, that it is a product of vegetables. 
VOL. IV. o o 
