THE OLD RED SANDSTONE. lll 
succession — now sandstone, now limestone, now stratified 
clay. But though intimately acquainted with these lower 
rocks for more than a hundred fathoms from their base up- 
wards, and with the upper rocks on both sides the ichthyo- 
litic bed for more than a hundred feet, there was an inter- 
vening hiatus, whose extent at this period I found it impos- 
sible to ascertain. And hence my uncertainty regarding 
the place of the ichthyolites, seeing that whole formations 
might be represented by the occurring gap. On the Moray 
Frith side, where the sections are of huge extent, a doubtful 
repeat in the strata at one point of junction, and an abrupt 
fault at another, cuts off the upper series of beds to which the 
organisms belong, from the lower to which the great conglom- 
erate belongs. On the Cromarty Frith side the sections are 
mere detached patches, obscured at every point by diluvium 
and soil; and, in conceiving of the whole as a continuous 
line, with the Lias a-top and the granite group at the bottom, 
I was ever reminded of those coast-lines of the ancient geog- 
raphers, where a few uncertain dots, a few deeper markings, 
and here and there a blank space or two, showed the blended 
results of conjecture and discovery — whether they give a 
Terra Incognita Australia to the one hemisphere, or a 
North-Western passage to the other. The ichthyolites in a 
section so doubtful might be regarded as belonging to either 
the Old or the New Red Sandstone — to the Coal Measures, 
or to the Mountain Limestone. All was uncertainty. 
One remark in the passing: it may teach the young geolo- 
gist to be cautious in his inferences, and illustrate, besides, 
those gaps which occur in the geological scale. I had now 
discovered the ichthyolite beds in five different localities ; in 
one of these — the first discovered —there is no overlying 
stratum ; it seems as if the bed formed the top of the forma- 
