^Hap. hi 
VOLCANIC CYCLES 
29 
such as inelaphyre, augite-porphyrite, and gabbro-diabase, having from about 
44 to about 52 per cent of silica. These were followed by rocks with a 
silica-percentage ranging from about 50 to 61, including some characteristic 
^^orwegiau rocks, like the rhomben-porpliyry. The acidity continued to 
increase, for in the next series of eruptions the silica-percentage rose to 
between 60 and 67, the characteristic rock being a ipiartz-syenite. Then 
«aine deep-seated protrusions of liighly acid rocks, varieties of granite, 
containing from 68 to 75 per cent of silica. The youngest eruptive 
’basses in the district show a complete change of character. They are 
basic dykes (proterobase, diabase, etc.).^ 
The same author institutes a comparison between the post-biliirian 
eruptive series of Christiania and that of the Triassic system in the Tyrol, 
*rnd believes that the two cycles closely agree." 
Durino’ Tertiary time in Central Prance more than one cycle may le 
loade 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, phonohtes 
®rui basalts. The Pliocene eruptions showed a reversion to the inter- 
ruediate types of augitic andesites and trachytes, followed by abundant 
basalts, which continued to be poured forth in Pleistocene time.'* 
Further north, in Auvergne, where the eruptions come down to a later 
period, the volcanic sequence appears to have been first a s^omewhat acn 
g’-oup of lavas (trachytes or domites), followed by a_ series^ of basalts, then 
andesites and labradorites, the latest outflows again consisting oi basalts. 
Not less striking is the succession of lavas in the Yellowstone region, 
described by Mr. Iddings. The first eruptions consisted of andesites. 
These were followed by abundant discharges of basalt, succeeded by later 
o*itflows of andesite, and of basalt like that previously erupted. After a 
Puriod of extensive erosion, occupying a prolonged interval of tune, volcanic 
energy was renewed by the eruption of a vast flood of rhyolite, after winch 
eanie a feebler outflow of basalt that brought the cycle to a close, thougli 
§eysers and fumaroles show that the volcanic fires are not yet entirely 
extinguished below.® 
But not only is there evidence of a remarkable evolution or succession 
erupted material within the volcanic cycle of a single geological period, 
^ne of the objects of the present work is to bring forward proofs that such 
* Eruplivqest. Kristianmjeb., 1895. , . , 
' Op. cit. He supposes in eaeli case the pre-existenco of a parent magma ^ 
'"'■aptive series started and wliich had a silica-percentage of about 64 or 65. In this difficult 
it is of the utmost iiiiportanee to accumulate fact before proceeding to speculation 
, ■' M, Boiile, “Description Geologique du Velay,” -Buff. Carle. OM. Fram’c, 1892. Hus authoi 
^raws special attention to the evidence for the alternation of basic and mure acid mateiial in tl 
Mtiary eruptions of Central France. 
■* M. Michel Levy, LnU. Soc. Giol. France, 1890, p. 704. , 1 , 
Journal of rjloqy. Chicago, i. (1893) p. 606. See also this author s 
“Electric Peak and Sepulchre Mountain,” mh Ann. Rep. U.S. Gcol. Surrey (1890-91X and 
7' H. W. Turner on “The Succession of Tertiary Volcanic Rocks 111 the Sierra Nevada oi IN 
■^’Uorica,” 14 ft Ann. Itcp. U.S. Gcol. Surrey (1892-93), p. 493. 
' North 
