BY R. M. JOHNSTON, F.LS. 91 
tieasurement of the “personal equation of error,” whether 
due to enthusiasm or preconception, than in cases where the 
imagination is apt to read or project hidden foregone causal 
conclusions or anticipations into those facts of evidence, 
which, taken by themselves, are readily adaptable to any one 
of many possible interpretations. There is no field of 
geological observation where there is more avidity shown 
in drawing hasty inferences, and forming generalisations from 
imperfect data, than in that section which concerns itself 
with the occurrence, dynamical effects, and hypotheses of 
causation, in respect of former glacial action. In no field is 
there such assurance expressed, based upon partial or imper- 
fect data, and in the face of the widest divergence of opinion 
in the interpretation of the same facts. Where sympathies 
are too strongly enlisted on behalf of a glacial or any other 
theory, they are apt to disarm the true critical faculty of the 
observer. He is apt to infer, too readily, that all rounded 
bosses and smoothed rock surfaces in the vicinity of old 
shingle beds are veritable voches-moutonnées,and that all shingle 
beds are moraines ; and under this preconception he is some- 
times not critical enough to distinguish the difference between 
the unequal wearing away of the laminations of polished 
stones derived from schistose rocks and the veritable s¢riz of 
ice action. 
He is apt to magnify one of the elements which, with other 
inks, are necessary to form the complete chain of proof—as 
itself the only element which may constitute pvoof in favour 
of a conclusion. As evidence of this partiality, we sometimes 
hear of the discovery of a single striated stone put forward as 
constituting the only veal proof of the occurrence of former 
glacial action. Yet the occurrence of huge perched blocks or 
erratics, many tons in weight, of a rock foreign to the imme- 
diate neighbourhood, resting on a recent accumulation of a 
well-known rock—loam, clay, ¢ cravel, or peat—although now 
devoid of either polished surfaces, scratches, or grooves, may, 
of itself, afford more unmistakable evidence in proof of ice 
action of a certain age, that any number of polished, striated 
and grooved stones, taken from a tumbled drum of waterworn 
stones and clay ; for the present accumulation in which the 
striated stones occur may not have been formed by ice action, 
although some of its contents may have been derived, imme- 
diately, from former moraine stuff; and even should the 
striated and polished blocks be now found in a veritable 
moraine, they do not form absolute proof that the ice mark- 
ings, or at least all of them, were actually caused by the 
glacier which formed the moraine in which they are now 
found ; for in many of the Scotch fiords or sea-loch basins— 
if we accept the theory of an inter-glacial period—we must 
