164 Professor Jameson’s Speculations on the Formation 
opal, wood-opal, and menilite. The menilite and wood-opal are 
the most modern of these, — the first occurring imbedded in the 
adhesive slate of the Paris formation, the other in tuffaceous 
rocks, of the nature of trachyte. The opals are found some- 
times 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 Iceland. From the magnitude and 
abundance of these springs, in many regions of the earth, and 
the quantity of siliceous matter they deposit, we can form a ge- 
neral estimate 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 form- 
ing in the mineral kingdom ; but one of the most interesting fea- 
tures 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 considerable abundance in se- 
veral 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 contains 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 resem- 
blance 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. 
Hornstone. — We shall next trace the distribution and for- 
mation of hornstone. This mineral, which, in its pure state, is 
principally composed of silica, occurs in considerable abundance 
in several primitive rocks. It appears also in rocks of the tran- 
sition-class, and is associated with different secondary rock for- 
mations. Wood, penetrated with hornstone, occurs occasionally 
in alluvial strata, as in clays and sands of various kinds, and ex- 
hibiting such characters, as shew that the petrifaction or pene- 
tration of the wood with the hornstone, had taken place in it af- 
ter it was enveloped in the clays and sands. Like opal, hornstone 
seems sometimes to be a product of vegetable origin ; for the spe- 
cimen which I now exhibit to the Society is a variety of wood- 
