OF VOLCANIC ROCKS. 



which will be more fully mentioned in other pages, they appear to owe their present 

 positions to the action of general planetary processes, and to reveal by their own 

 nature that of the mineral matter participating in the original composition of the 

 globe, and by their order of succession, the mode in which the same is arranged 

 beneath the theater of those changes which since a remote period have been taking 

 place on its surface. Only presumptive evidence can be adduced in favor of this 

 common and yet much disputed theory. The probability of its approximating the 

 truth could hardly be better established by any other evidence than by the proof that 

 all eruptive rocks of the globe, taking their historical part into account, are capable 

 of being brought into a natural system ; or, to express it more correctly, that they 

 form among themselves a natural system, the laws of which we may be capable of dis- 

 covering. Considering this in its widest bearing, as embracing all the definite correla- 

 tions of eruptive rocks, and being indeed their philosophical expression, we may 

 expect that it will make us acquainted with the history of one great feature in the 

 development of the globe. Bearing in their own character and system the imprint of 

 their origin, the eruptive rocks will, by their nature itself, allow well-founded con- 

 jectures as to the interior structure and composition of the earth. This, then, 

 together with a more perfect understanding of everything connected with the agencies 

 working below the surface of the globe, would be the philosophical use of the natural 

 system of eruptive rocks. 



Only initiatory steps can be taken at the present time towards the establish- 

 ment of this system. I have confined myself in this essay to an attempt at classifying, 

 in a way as natural as experience will allow, the " volcanic rocks," that is, the eruptive 

 rocks of Tertiary and Post-Tertiary ages. The term "volcanic rocks," has been chosen, 

 because the rocky matter ejected by active volcanoes belongs altogether to this class, 

 and because almost every kind of rock, generated by eruptive activity during the 

 period indicated, has partially forced its way through volcanic vents. We must keep 

 the two-fold mode of occurrence of volcanic rocks clearly separated in our minds. We 

 see them at the present day flowing from craters in the shape of lava, or being thrown 

 out as scoria and rapilli ; and there is abundant evidence that their mode of origin 

 was very frequently the same in past ages. But in other places, it is perfectly clear that 

 volumes of matter of the same kind have been forced to the surface through extensive 

 fissures and accumulated above them in elongated ranges, when the origin of the out- 

 breaks cannot be ascribed to volcanic activity. These eruptions are evidently similar' 

 in nature to those by which the greater part of the granite, syenite, or quartzose por- 

 phyry ascended to the surface in ancient times. We shall distinguish the two modes 

 of eruptions, for the use in this present paper, as "volcanic eruptions" and "massive 

 eruptions," and shall dwell in the sequel more fully on the difference between both 

 manifestations and their probable causes. Then, too. the reasons will be mentioned 

 which justify the uniting of all recent eruptive rocks into one separate class. 



No other class of eruptive rocks offers greater difficulties for a systematical 

 arrangement, as none presents throughout so great a number of varieties, and so many 

 accidental modifications of texture and mineral composition. On the other hand, their 



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