598 
ELEMENTS OF GEOLOGY. 
all monuments of the date of its development throughout a 
whole mountain chain, and all the labor and skill of the most 
practised observers are required, and may sometimes be at 
fault. I shall mention one or two examples of alteration on 
a grand scale, in order to explain to the student the kind 
of reasoning by which we are led to infer that dense masses 
of fossiliferous strata have been converted into crystalline 
rocks. 
Eocene Strata rendered metamorphic in the Alps.—In the 
eastern part of the Alps, some of the Palaeozoic strata, as well 
as the older Mesozoic formations, including the oolitic and 
cretaceous rocks, are distinctly recognizable. Tertiary de¬ 
posits also appear in a less elevated position on the flanks of 
the Eastern Alps; but in the Central or Swiss Alps, the 
Palaeozoic and older Mesozoic formations disappear, and the 
Cretaceous, Oolitic, Liassic, and at some points even the Eo¬ 
cene strata, graduate insensibly into metamorphic rocks, con¬ 
sisting of granular limestone, talc-schist, talcose-gneiss, mica¬ 
ceous schist, and other varieties. 
As an illustration of the partial conversion into gneiss of 
portions of a highly inclined set of beds, I may cite Sir R. 
Murchison’s memoir on the structure of the Alps. Slates 
provincially termed ‘^flysch” (see above, p. 278), overlying 
the nummulite limestone of Eocene date, and comprising 
some arenaceous and some calcareous layers, are seen to al¬ 
ternate several times with bands of granitoid rock, answer¬ 
ing in character to gneiss. In this case heat, vapor, or water 
at a high temperature may have traversed the more permea¬ 
ble beds, and altered them so far as to admit of an internal 
movement and re-arrangement of the molecules, while the 
adjoining strata did not give passage to the same heated 
gases or water, or, if so, remained unchanged because they 
were composed of less fusible or decomposable materials. 
Whatever hypothesis we adopt, the phenomena establish be¬ 
yond a doubt the possibility of the development of the meta¬ 
morphic structure in a tertiary deposit in planes parallel to 
those of stratification. The strata appear clearly to have 
been affected, though in a less intense degree, by that same 
platonic action which has entirely altered and rendered met¬ 
amorphic so many of the subjacent formations; for in the 
Alps this action has by no means been confined to the imme¬ 
diate vicinity of granite. Granite, indeed, and other plutonic 
rocks, rarely make their appearance at the surface, notwith¬ 
standing the deep ravines which lay open to view the inter¬ 
nal structure of these mountains. That they exist below at 
no great depth we can not doubt, for at some points, as in 
