OF VOLCANIC ROCKS. 9 
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 voleanoes 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 voleanic 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 ‘‘ volcanie 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 
(47) 
