OF VOLCANIC ROCKS. AT 
able to discover the laws of the natural system embracing their totality. In order to 
arrive at a more perfect understanding of this system, we must now attempt to exam- 
ine into the causes of those mutual relations. It would naturally occur to us that they 
would be implied in the circumstances which attended the generation of the eruptive 
rocks, and in the conditions in which they have been before arriving at the places they 
occupy at present. We have, therefore, to investigate the following questions : What 
was the nature of volcanic rocks before they arrived at the places which we now 
see them occupying? Where did they originate? By what agencies did rocks con- 
nected in widely separated places by simple and definite relations come to their 
positions among others which bear no such relations either to them or among each 
other? The importance of these questions for our subject, the attention which they 
have attracted through the whole history of geological science, and the great diversity 
of opinion prevailing in regard to them, will make it necessary to treat them more 
fully than might otherwise appear consistent with the objects of this paper. 
Voleanic action and massive eruptions, notwithstanding the similarity of the 
material produced by both, would appear, from the most cursory review of the 
phenomena connected with either of them, to differ to some extent, not only in regard 
to the causes to which they owe their origin, but also in regard to the position the 
matter occupied before its ejection. We have for this reason to keep again distinctly 
separated these two modes of manifestation of subterranean energy. In regard to 
massive eruptions, which will first occupy our attention, we can scarcely draw up any 
argumentation without enlarging on the entire range of eruptive rocks, and extending, 
at least partially, our views to them.” 
1. On the Origin of Massive Eruptions. 
In order to establish some positive premises available for drawing conclusions 
in regard to the origin of the massive eruptions of volcanic rocks, we reiterate the 
following facts, of which mention has partly been made in the foregoing pages: 
Ist. The eruptive (including the voleanic) rocks offer a great diversity of chemical 
composition ; but all the compounds represented by them are mutually connect- 
ed by simple and definite arithmetical relations as regards the figures which 
17 The following considerations are given notw ithout some hesitation, partly on account of the uncertain ground on 
which they have to move, and partly because some of their main features are, of necessity, only well known theories repro- 
duced, though, perhaps, under a somewhat different form. Yet, the establishment of the relations detailed in the preceding 
chapters, and other observations made of late years, may allow us to arrive at more satisfactory conclusions in regard to 
some weighty problems than could be done before, or, at least, to determine more precisely the only direction in which we 
have to look for their solution. It should be borne in mind that among the theories recently proposed upon the subjects 
specified above, there is not one which has not already had its prototype in the phantasmagorias of the time of the dawn of 
geological science, and that it is these which have been constantly reproduced, enlarged, diversified, remodeled according to 
the advance of science, and supported by continuous accumulation of evidence. Propositions which had been accepted as 
being beyond the necessity of proof, and which are still occasionally reproduced as axioms in popular works on geology, have 
been weakened, and not unfrequently overthrown, when facts newly revealed would withdraw their chief supports, but have, 
after some time, revived under new forms. In treating on a topie where the degree in which the results of our speculation 
appear satisfactory to us depends upon the degree of probability which we think we see in the theories arrived at, and of 
their faculty of explaining observed facts, and where we are in constant danger of making incorrect deductions from imper- 
fect premises, not enough can be done in the way of weighing the evidence by which different doctrines are supported ; and 
this is particularly necessary in reference to those theories which we are too much accustomed to consider as matters of fact, 
upon which further conclusions may be safely built. (85) 
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