502 GEOLOGY OF THE WESTERN DISTRICTS OF CANADA. 
follows the formation usually denominated Niagara shale, about eighty 
feet thick, consisting of a homogeneous stratified or laminated mass 
of bluish-grey, sometimes nearly black, argillaceous, arenaceous and. 
calcareous slatey rock, hard and solid in the bed, but decomposing and 
crumbling when exposed to the atmospheric influences. It seems to 
be devoid of fossils, except towards its junction with the underlying 
hard limestone, where it is plentifully charged with Pentamerus and. 
dtrypa. 
Lastly. the escarpment is capped by the Niagara limestone, (so 
called) a massive and very hard dark blue or more nearly black rock, 
the lower portions being in very thick solid beds, while towards the 
top the partings occur more frequently. This rock is magnesian and 
silicious in mineral character, and is highly bituminous, being known 
in many places to emit inflammable gas through the seams. Occasion- 
ally it is cavernous in structure, and is copiously interspersed with 
druses or cavities containing cale-spar, gypsum and sulphate of Stron- 
tian. I have been uunable to detect any fossil remains in this 
formation, although I believe they are not altogether wanting. It 
is over this rock that the great cataract is precipitated, and it forms 
from its hardness a species of coat of mail or armour of proof te 
resist the too rapid erosions of the torrent. 
Proofs of Retrocession.—It will serve at once to illustrate strikingly 
what may be called the mechanical properties of the strata we have 
been considering, and at the same time to show by a most remarkable 
example the value of geological evidence in regard to duration of 
time, if we take up at this stage the subject of the recession of the 
great Falls. 
It has long been a well known fact, that behind the mighty 
cataract there existed a vast cavern formed by the action of the water 
and air set in violent motion by the descending torrent upon the soft 
shales underlying the Niagara limestones; and this fact must have 
suggested to an enquiring mind the idea, that as the soft material 
became gradually undermined or excavated, the weight of the super- 
structure and impetus of the water must have caused the harder 
superincumbent rock from time to time to give way, and thus oceasion 
a. recession of the Fall in its position. In accordance with this idea, 
it is found from historic evidence, (which unfortunately in this point 
affords less corroboration to geological theories than in questions 
relating to the old world), that changes of the kind referred to had. 
