26 RICHTHOFEN—NATURAL SYSTEM 
small but distinct tabular crystals of oligoclase, fine grains of titanic iron, small, 
elongated columns of hornblende, sometimes mica, and in most varieties a few isolated 
crystals or rounded grains of augite. Frequently these different crystals are so small 
as to be no longer recognizable, except by the aid of the microscope. The rock is 
then similar in character to certain varieties of melaphyr, but may be distinguished 
by its vesicular texture, which is scarcely ever wanting in the hornblendic varieties. 
This family comprises vastly the greater portion of all andesitic rocks. 
Fam. 2. Augitic Andesite-—Two groups of varieties may be distinguished 
among the rocks of this family. Those of the first have a paste of an oil-brown color 
passing into black, a compact appearance, and ordinarily a microcrystalline tex- 
ture which passes into that of porcelain. Vesicular inflation does ordinarily not 
occur. Crystals of a monoclinohedric feldspar (probably labrador) are almost invari- 
ably enclosed, and frequently accompanied by hornblende and augite. For the rocks 
of this group the name ‘ trachydoleritte” has particularly been applied. The other 
group comprises certain varieties for which the name ‘‘ anamesite”’ has not rarely been 
used, and which are of dark-gray colors. Their texture is in most cases vesicular. 
The enclosed minerals are the same as with the other group; but, while for this 
the feldspathic ingredient is more characteristic, and frequently alone present in large 
crystals, the rocks of the second group are distinguished by the predominance of well 
formed crystals of augite and hornblende, while those of feldspar are often so small 
that they cannot be distinguished by the eye. The rocks of both these groups contain 
titanic iron in larger quantity than those of the first family, and are of greater specific 
gravity. Olivine enters occasionally into their composition, in very subordinate quan- 
tity. Geologically, the rocks of this family are closely allied to hornblendic andesite. 
They have succeeded it in age, and are limited in their geographical distribution to those 
places where hornblendic varieties had been ejected before, frequently intersecting and 
overlying them. Notwithstanding the resemblance which they bear to certain varie- 
ties of basaltic rocks, they appear to be never associated with them geologically. 
OrpER FirrrH—Basatr. 
No one of the names applied to volcanic rocks is of equal antiquity with that of 
“basalt,” and with none there has ever been less change of opinion, and uncertainty in 
respect to its application. It may be inferred from this fact that the rocks comprised 
by that name are very distinct in character and little liable to variation, which is indeed 
true for the typical rocks of the order. But there have to be associated in the same 
order with them certain other rocks, which, though nearly related to basalt in regard to 
their chemical and mineral composition, differ from it in the more conspicuous external 
characters. 
Mode of Geological Occurrence.—Basaltic rocks are more independent than those 
of any of the foregoing orders (perhaps with the exception of propylite), in respect 
to their epoch of ejection as well as to their geographical distribution. In the order 
of time they succeeded next to rhyolite, but locally, both are usually separated. 
Basalt occurs always in the neighborhood of more ancient volcanic rocks, but the cases 
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