4 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
by many of the mountain-tops and slopes in our hilly 
regions. The rocks were often more or less concealed 
below masses of coarse angular fragments of all shapes 
and sizes. The hill-tops frequently looked as if they had 
been subjected to the battering action of some mighty 
hammer, which had smashed and shattered the rocks to a 
considerable depth; so that, if we wished to get at the 
solid and undisturbed parent-mass, we should first have to 
clear away many feet, and even sometimes many yards, 
of more or less loose debris. The slopes of such debris- 
capped mountains were invariably clothed with long 
sloping taluses of similar fragments, which swept down at 
a high angle to the valleys, and at the base of these slopes 
large blocks and isolated masses of rock were of common 
occurrence. No one who should examine these phenomena 
could ior a moment doubt that they owed their origin to 
the action of the atmospheric agents. Dr Geikie then 
described, in a somewhat detailed manner, the mode in 
which the rocks were broken up by the disrupting force 
of ice. Water found its way into the crevices of the 
rocks, and, being frozen there, the joints were gradually 
widened by the expansion of the ice, again and again 
repeated. When this action took place on a flattish hill¬ 
top the rocks were simply disrupted, and the separate 
fragments pushed asunder. But upon the verge of preci¬ 
pices, and upon steep slopes, the disrupted fragments 
were shot downwards, as soon as thaw set in. There 
were other ways in which rock-debris or natural rubbish- 
heapsjwere formed. Strata were often undermined by 
the action of water, and large masses of rock, deprived of 
their support, tumbled down in ruins. This could be seen 
at the base of sea-cliffs, and along the margins of streams 
and rivers. Then, again, some kinds of rock which were 
morejpr less soluble in water were liable, under certain 
circumstances, to be disjointed and broken up. Lime¬ 
stone, for example, was dissolved by the action of acid¬ 
ulated water working its way downwards through the 
natural fissures of the rock. In process of time these 
fissures were widened by this solvent action, and con¬ 
verted into irregular channels and tunnels. This was the 
origin of most of our limestone caverns. Water continu¬ 
ing to percolate down into such caves gradually loosened 
the limestone that formed the roof, and now and again 
large and small fragments of the rock, losing cohesion, 
fell to the ground. Another cause for the origin of rock- 
debris was to be found in the peculiar geological structure 
of certain masses of strata, which were so arranged as to 
render them liable to sudden and wholesale demolition. 
When a mountain was built up of a series of porous and 
non-porous strata, arranged in alternate layers, dipping 
into the valleys at such a low angle that the edges of the 
beds were exposed upon the mountain-slopes, such a 
mountain might at any moment be destroyed. Dr Geikie 
then referred to several remarkable examples of such 
catastrophes. In the case of the Bossberg, in Switzerland, 
the destruction was due to the fact that long-continued 
rains, soaking down through porous beds above, were 
arrested by beds of non-porous clay, which, however, 
became softened to such a degree that the mountain-mass 
of strata that rested upon them slid forward upon them, 
and rushed down into the valley. After describing yet 
other modes in which natural rubbish-heaps were formed, 
Dr Geikie went on to remark that all the phenomena 
referred to were more or less exceptional, and that the 
agent which effected the greatest results was frost. Some 
of the other agents he had described could only work 
under certain geological conditions;—others, again, were 
somewhat limited in their action, and tended to remove 
the rubbish-heaps which they themselves had accumulated. 
But the action of frost in a country like ours was, he 
might say, general. It affected every part of the land, 
but of course the amount of work it performed was very 
variable. Its results were most conspicuous in mountain- 
regions, where frosts were not only more frequent, more 
intense, and more prolonged, but where the physiographi- 
cal conditions of the surface lent their aid in the most 
effective manner. The rock-debris gathered to the greatest 
thickness upon slopes at the base of a rocky precipice. 
This was natural, for the steep rocks above, shattered by 
frost, showered their debris downward. But on flat hill¬ 
tops the time must come when the formation of rock- 
debris would terminate. The rock would only be acted 
upon to as great a depth as the frost could penetrate. 
Some account of the frost-riven debris of other countries 
was then given, more especially of the Swiss Alps, and 
northern regions of Europe and North America. It was 
remarkable that many parts of our own country were 
covered with sheets of debris which had apparently long 
ago ceased to accumulate, and these sheets occurred not 
only upon comparatively low ground, but even in mountain- 
regions. The angular fragments were grown over now 
with lichen and heath, and even with natural wood, and 
in every feature betrayed the marks of great antiquity. 
And not only so, but they occurred in positions to which 
loose blocks detached from the rocks at higher levels 
could not possibly have rolled. It was hard enough to 
account for the presence of such sheets of ancient angular 
debris in a country like Scotland, but it was more difficult 
still to explain the presence of similar sheets of angular 
debris at low levels in the south of England, in 
Northern France, in Southern Spain, and at many 
places upon the borders of the Mediterranean. After 
