28 RICHTHOFEN—NATURAL SYSTEM 
Fam. 3d. Leuettophyre—Rocks of porphyritic texture, crystals of augite and 
leucite being imbedded in the paste. 
It will be noticed that no place has been assigned in this classification to phon- 
olite. As regards those rocks for which this name was first proposed, our knowledge 
is still quite limited. It appears that, notwithstanding their comparatively rare occur- 
rence, they form a distinct natural group closely allied to basalt; but they are so dif- 
ferent from true basalt as regards lithological characters, that they should not be 
classified with it before further observations will have determined their real position 
in reference to the natural families of volcanic rocks. The name phonolite has, how- 
ever, been so much extended in its application, that this task is not so easy to accom- 
plish. Some external properties, easy of recognition to superficial observation, such 
as a certain tabular structure, lithoid texture of the paste, with small, bright crystals 
of feldspar enclosed, and the peculiarity of ringing by a blow of the hammer, which 
have been often considered as the characteristic features of phonolite, are just as com- 
mon with certain varieties of trachyte and rhyolite, and even with some of propylite. 
It must be ascribed to this reason, that the name phonolite has been used for the 
designation of rocks which bear an accidental resemblance to true phonolite, but are 
distinct from it in nature. It has probably been oftener applied to rocks belonging to 
the trachytic order than to such as have the distinguishing features of those varieties 
for which the name has been first used. 
CorRELATION OF THE Five Orpers oF Votcanic Rocks. 
In the foregoing pages I have attempted to lay down the outlines of a classifi- 
cation of the volcanic rocks by natural principles, and to apply a nomenclature which 
should be appropriate to these, and embrace, at the same time, the most current of 
existing names. It is the next object of this paper to prove that these rocks are 
mutually connected by definite relations, and that their totality, in virtue of this 
property, forms actually what may be called a system in nature, and that the form into 
which we have tried to bring it, imperfect though it must be, is an approach towards 
its expression. In order, therefore, to fully realize the philosophy of the natural 
system, we have to contemplate the relations which, firstly, the rocks of the different 
orders offer mutually among themselves, and by which, secondly, they are connected 
as an entire class with ancient eruptive rocks ; while we will have, thirdly, to examine 
into the mode of origin of volcanic, and of eruptive rocks in general, in order to 
establish the nature of their fundamental difference from sedimentary and metamor- 
phic rocks. Our task is thus three-fold. The present chapter will be devoted to the 
first order of relations. They may be considered from several points of view, the 
more important of which are: chemical and mineral composition, geographical distri- 
bution, and all those complex relations which may be comprehended in the term 
‘‘mode of geological occurrence.” We will contine ourselves to the last point of view. 
But even with this restriction, we can only trace general outlines. 
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