28 
VOLCANIC PRODUCTS 
BOOK I 
ture and composition iu a single mass of lava is another problem presented 
by the remarkable secpxence of different types of lava which are erupted 
within a given district during a single volcanic period. Nearly thirty years 
ago Tlaron von Pdchthofen drew attention to the sequence of volcanic 
materials erujited within the same geographical area. He showed, more 
especially from observations in Western America, that a definite order of 
appearance iu the successive species of lava could be established, the earliest 
eruptions consisting of materials of an intermediate or average composition, 
and those of subsequent outflows becoming on the whole progressively more 
acid, but finishing by an abrupt transition to a basic type. His sequence was 
as follows : 1. 1’ropylite ; 2. Andesite ; 3. Trachyte ; 4. Ehyolite ; 5. Basalt.^ 
This generalisation has been found to hold good over wide regions of 
the Old World as well as the New. It is not, however, of universal appli- 
cation.^ Examples are not uncommon of an actual alternation of acid and ■ 
basic lavas from the same, or at least from adjacent vents. Such an alterna- 
tion occurs among the Tertiary eruptions of Central France and among those 
of Old lied Sandstone age iu Scotland. 
The range of variation 'in the natrrre of the eruptive rocks during the 
whole of a v'olcanic period in any district may be termed “ a volcanic cycle.” 
In Britain, where the records of many volcanic periods have been preserved, 
a numlrer of such cycles may be studied. In this way the evolutioir of the 
subterranean magma during one geological age may be compared with that 
of another. It will be one of the objects of the following chapters to trace 
out this evolution iu each period where the requisite materials for the 
purpose are available. We shall find that back to Archaean time a number 
of distinct cycles may be observed, differing in many respects from each 
other, but agreeing in the general order of development of the successive 
eruptions. Leaving these British examples for future consideration, it may 
be useful to cite hero a few from the large series now collected from the 
European continent and North America.^ 
Among the older rocks of the European continent. Prof. Brogger has 
shown that in the Christiania district the eruptive rocks which traverse the 
Cambrian and Silurian formations began with the outburst of basic material 
^ Tran.i. Acad. California., 1868. Prof. Iddings’ Jcnirn. Geol., vol. i. (1893), p. 606. 
- See Prof. Brogger, “ Die Eruptivgesteine des Kristianiagebietes, ” part ii. (1895), ji. 175 ; Zcitsch. 
Kri/st. mid Mineral, vol. xvi. (1890) p. 83. This author wouhl, from thus ]iohit of view, draw 
a distinction between rocks which have consolidated deep within the earth and those which have 
flowed out at the surface, since he thinks that we are not justified in applying our experience of 
the order of sequence in the one series to the other. Yet there can he no doubt that in many old 
volcanic districts the masses that may be jwesuined to have consolidated at a gi'cat depth have 
lieen iu unbroken connection with nia.sses that reached the surface. These latter, a.s Prof. Iddings 
has urged, fiu-nish a much larger body of evidence than the intrusive sheets and bosses. 
^ Prof. M. Bertrand iu a suggestive jtaper published in 1888 dealt with the general order of 
ajipearanoe of eruptive rocks in different provinces of Europe. But the materials then at his 
command probably did not warrant him in offering more than a sketch of the subject, Bull. Soc. 
Geol., France, xvi. p. .573. In the .same volume there is a paper by JI. Le Veirier, who announces 
his opinion that the eruption of the basic rocks takes jdacc in times of terrestrial calm, while 
that of the acid rocks occurs in periods of great disturbance, up. cU. p. 498. Compare also 
Prof. Briigger, I>ie Eruptivgesteine dcs Kristiaiiiagchicles, ii. p. 169. 
