The same author institutes a comparison between the post-Silurian eruptive series of Christiania and that of the Triassic system in the Tyrol, and believes that the two cycles closely agree.[12]
[12] Op. cit. He supposes in each case the pre-existence of a parent magma from which the eruptive series started and which had a silica-percentage of about 64 or 65. In this difficult subject it is of the utmost importance to accumulate fact before proceeding to speculation.
During Tertiary time in Central France more than one cycle may be made out in distinct districts. Thus in the Velay, during the Miocene Period, volcanic activity began with the outpouring of basalts, followed successively by trachytes, labradorites and augitic andesites, phonolites and basalts. The Pliocene eruptions showed a reversion to the intermediate types of augitic andesites and trachytes, followed by abundant basalts, which continued to be poured forth in Pleistocene time.[13]
[13] M. Boule, "Description Géologique du Velay," Bull. Carte. Géol. France, 1892. This author draws special attention to the evidence for the alternation of basic and more acid material in the Tertiary eruptions of Central France.
Further north, in Auvergne, where the eruptions come down to a later period, the volcanic sequence appears to have been first a somewhat acid group of lavas (trachytes or domites), followed by a series of basalts, then by andesites and labradorites, the latest outflows again consisting of basalts.[14]
[14] M. Michel Lévy, Bull. Soc. Géol. France, 1890, p. 704.
Not less striking is the succession of lavas in the Yellowstone region, as described by Mr. Iddings. The first eruptions consisted of andesites. These were followed by abundant discharges of basalt, succeeded by later outflows of andesite, and of basalt like that previously erupted. After a period of extensive erosion, occupying a prolonged interval of time, volcanic energy was renewed by the eruption of a vast flood of rhyolite, after which came a feebler outflow of basalt that brought the cycle to a close, though geysers and fumaroles show that the volcanic fires are not yet entirely extinguished below.[15]
[15] Journal of Geology, Chicago, i. (1893) p. 606. See also this author's excellent monograph on "Electric Peak and Sepulchre Mountain," 12th Ann. Rep. U.S. Geol. Survey (1890-91), and Mr. H. W. Turner on "The Succession of Tertiary Volcanic Rocks in the Sierra Nevada of North America," 14th Ann. Rep. U.S. Geol. Survey (1892-93), p. 493.
But not only is there evidence of a remarkable evolution or succession or erupted material within the volcanic cycle of a single geological period. One of the objects of the present work is to bring forward proofs that such cycles have succeeded each other again and again, at widely separated intervals, within the same region. After the completion of a cycle and the relapse of volcanic energy into repose, there has been a renewal of the previous condition of the subterranean magma, giving rise ultimately to a similar succession of erupted materials.
If we are at a loss to account for the changes in the sequence of lavas during a single volcanic cycle, our difficulties are increased when we find that in some way the magma is restored each time to somewhat the same initial condition. Analogies may be traced between the differentiation which has taken place within a plutonic intrusive boss or sill and the sequence of lavas in volcanic cycles. It can be shown that though the original magma that supplied the intrusive mass may be supposed to have had a fairly uniform composition deep down in its reservoir, differentiation set in long before the intrusive mass consolidated, the more basic constituents travelling outwards to the margin and leaving the central parts more acid. If some such process takes place within a lava-reservoir, it may account for a sequence of variations in composition. But this would not meet all the difficulties of the case, nor explain the determining cause of the separation of the constituents within the reservoir of molten rock, whether arising from temperature, specific gravity, or other influence. This subject will be further considered in connection with intrusive Bosses.