28 RICHTHOFEN — NATURAL SYSTEM 



Fam. od. Leucitophyre. — 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 Orders of Volcanic 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 maybe 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 confine ourselves to the last point of view. 

 But even with this restriction, we can only trace general- outlines. 



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