Vol. 58.] ANNIVERSARY ADDRESS OF THE PRESIDENT. Tal 
appearance of the inequalities of an area of closed drainage beneath 
terrestrial accumulations of enormous thickness, represented by leess, 
sand-dunes, dry deltas, screes, and the subaqueous deposits of salt- 
lakes. If similar conditions prevailed in the past, two strongly 
contrasted types of unconformity may be expected to occur: one 
characterized by marine deposits resting on a plain of denudation, 
and the other by continental formations resting on an old uneven 
Jand-surface. The North-west of Scotland and the South-west of 
England present us with admirable examples of both types of un- 
conformity. The base of the Torridonian is an old, uneven land- 
surface, whose mountains and valleys he buried under huge accumu- 
lations of sandstone, conglomerate, and breccia ; but the base of the 
Cambrian is a smooth plain, whose inequalities are due to the later 
earth-movements. Similarly, in the West of England the base of 
the New Red Sandstone is uneven, whereas that of the marine 
Cretaceous is smooth and approximately horizontal. 
The point at which we have arrived may now be briefly sum- 
marized. The sedimentary rocks form a well-defined natural 
group. ‘They arise in consequence of complex chemical, physical, 
and organic processes, depending on reactions between the hydro- 
sphere and atmosphere on the one hand and the lithosphere on the 
other. 
CRYSTALLINE ScHiIsts AND Mrramorpuic Rocks. 
I now approach the most difficult portion of my task—that which 
deals with the history of opinion on metamorphic rocks and the 
crystalline schists. It is scarcely possible even to enumerate all 
the diverse views that have been expressed as to the origin of 
the crystalline schists. Some geologists have maintained that they 
are portions of the original primitive crust; others that they 
are chemical precipitates from a primordial ocean; others that they 
are the result of a peculiar kind of metamorphosis, diagenesis, 
acting on the chemical precipitates of such an ocean ; others that 
they are due to metamorphic processes operating upon ordinary 
sediments without seriously disturbing, or in any way obliterating, 
the original order of stratification; and others that they are the 
result of dynamic and thermal agencies affecting complex systems 
of igneous and sedimentary rocks. On looking back at the nineteenth 
century we see all these ideas, and many others, struggling for 
existence. Natural selection has been at work, to the detriment, as 
it seems to me—and I make no claim to the position of impartial 
WOES EVIE, g 
