Bituminoid Cement of the Cuithness Flagstones. — 317 
In approaching an inquiry of this kind, probably no 
eeologist would venture to propound an explanation of the 
facts in any form except that of a temporary working 
hypothesis, which he would regard more as a suggestion of 
what may be possible, rather than as a final answer to a 
question which, whenever it comes to be answered definitely, 
must be answered by a chemist. Up to the present no 
chemical geologist has even attempted a solution of the 
difficulty ; in the mean time, and until someone does do so, 
we may endeavour to form some kind of idea for ourselves, 
by making the best we can of what evidence we can gather 
in the field. With this object in view, we need first to review 
the more salient petrographical characters of each of the larger 
divisions of the Old Reds, noting, especially, what charac- 
teristics accompany each type, alike in the cases where 
bituminoids are present and where they are absent. 
It is now generally known amongst geologists that the 
Devonian Period in the northern parts of the kingdom was 
one during which continental instead of marine conditions 
obtained, and that it was here also a period characterised by 
important terrestrial disturbances. The general nature of 
these has been indicated in my Address on the “ Age of the 
Earth,” printed in the present volume. It will suffice here 
to summarise the general conclusions upon this point. Owing 
to the terrestrial undulations being of greater amplitude, 
shorter wave-length, and higher frequency, than is usual in 
such earth-movements, and owing also to the fact that these 
undulations progressed from south to north, important up- 
heavals and denudations of the strata went on concurrently 
with depressions of the surface, and consequent deposition, 
in areas closely adjacent. The earliest formed Old Red 
(the Lanarkian) appears thus to have furnished materials for 
the succeeding (or Caledonian) Old Red. This in its turn, 
as the upward phase of undulation progressed northward, 
may have, in like manner, furnished materials for other sub- 
divisions higher still: successively-newer strata thus being 
formed as the terrestrial movements progressed from south 
to north. Two important unconformities result from these 
complicated movements. 
