90 GEOGNOSY AND GEOLOGY. 
must suppose one ingredient to have been completely formed before another 
was added to it, and forced by accidental agencies into a combination. 
Examples of heteronomic rocks are afforded by sandstone, grauwacke, and 
various other conglomerates. Conglomerates consist of two principal 
portions, one combining, and the other combined. The portion combined 
is that which we suppose to have been completely formed: it generally 
presents the appearance of fragments of different isonomic rocks, rounded 
by water into pebbles of various sizes. The combining portion is an earthy, 
finer substance, cementing the first into a compact mass. This is accord- 
ingly termed the cement. The following survey of the petrographical 
system will, it is hoped, serve to render the distinctions between isonomic 
and heteronomic rocks more intelligible, as well as to introduce the subject 
of Oreography, which is to be treated of subsequently. 
Il. SPECIAL PETROGRAPHY. 
A. Isonomic Rocks. 
These present themselves to us under various conditions, and it is a 
matter of great interest to study the connexion between the different 
formation stages, and the probable or certain origin of rocks. With respect 
to the origin of many rocks we know little or nothing; as regards others, 
such as those which we see forming under our own eyes, we can speak with 
entire confidence. We need only refer to the lavas which burst forth as 
molten masses from the bowels of the earth, or to the different deposits from 
waters. Analogies, such as are of common occurrence, must furnish the 
key to those rocks with regard to whose formation we cannot speak with 
positive certainty. Take porphyry as an illustration, a rock in whose 
imperfectly crystalline, or entirely amorphous substance, we often find single 
crystallized particles, and even the most beautiful individual crystals. 
Analogies to this character are to be found in many processes of art. If 
melted glass be cooled very slowly, it is not a rare occurrence for single 
crystals to separate in the compact amorphous mass; and a similar pheno- 
menon is observed in the slags produced in different metallurgical operations. 
The same is the case with respect to lavas in those parts of a lava current 
where a gradual cooling has taken place. When we find precisely the same 
circumstances in the older rocks, why may we not ascribe them to a similar, 
if not actually identical cause ? When we see that during an exceedingly 
slow cooling of a vitreous slag, a crystalline granular solid is produced, 
possessing the same texture as many plutonic rocks, why are we not 
entitled to conclude that the latter have been produced in a similar 
manner ? 
The principal distinctions to be made, with respect to the variety of 
crystallization of isonomic rocks, are the perfectly crystalline, the semi- 
crystalline, and the imperfectly crystalline. These modifications occur 
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