THE ORIGIN OF MOUNTAINS. 
77 
cases the extension or tear- 
ing is due, not to a genera! 
extension of the rock, but 
to lateral thrust. 
Fig. 1 9 represents a piece 
of nummulitic limestone in 
which the rock has not only 
been fractured along the 
lines a b, but two sides of 
the vein a have been evi- 
dently displaced. At a later 
date another fracture has 
taken place along the line c d. 
Some rocks have been 
so kneaded and ground to- 
gether that in many places it 
is rare to find a cubic milli- 
metre next its original neigh- 
bours.* In many places frag- 
ments and wedges of one 
formation have been forced 
into another. 
In the Tertiary slates 
of the Sernfthal at Platten- 
* Heim, Mech. d. Gi'lirgsb,, 
vol. i. 
Fig. 18. — Stretched and broken Eclemnitcs, J /. 2 size. A, Belemnites hastilis, slightly broken, Frete dc Saille. B, Specimen 
much drawn out. C, Section at n. 
