VI | MOUNTAINS AT 
those in the interior, and consequently they 
could not lie flat between A” and B”, but 
must be thrown into folds, commencing along 
any line of least resistance. Sometimes in- 
deed the strata are completely inverted, as 
in Fig. 19, and in other cases they have 
been squeezed for miles out of their original 
position. This explanation was first, I be- 
lieve, suggested by Steno. It has been re- 
cently developed by Ball and Suess, and espe- 
cially by Heim. In this manner it is probable 
that most mountain chains originated." 
The structure of mountain districts confirms 
this theoretical explanation. It is obvious 
of course that when strata are thrown into 
_folds, they will, if strained too much, give 
way at the summit of the fold. Before doing 
so, however, they are stretched and conse- 
quently loosened, while on the other hand the 
strata at the bottom of the fold are compressed : 
the former, therefore, are rendered more sus- 
ceptible of disintegration, the latter on the 
contrary acquire greater powers of resistance. 
1See especially Heim’s great work, Unt. ti. d. Mechanismus 
der Gebirgsbildung. 
