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 audesitic rocks. 



Fam. 2. Avgitic Andesile. — 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 " trachydolerite " 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 famil}', 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. 



Order Fifth — Basalt. 



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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