PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. Cxli 
to bring tbe truth home to the minds of others who have not thought 
the matter out for themselves. We need not wonder, then, that it is 
barely a century since this theory of the gradual development of the 
sedimentary rocks, through the action of familiar forces, was first 
broached by James Hutton.* 
Stated in their barest outline, the processes of denudation are 
carried on as follows. Heat, frost, moisture, and acids contained 
in the atmosphere, loosen the surface particles of a rock-mass; 
the rain washes these loosened particles into the nearest stream; 
the stream bears them along in its current towards the sea, and 
as it does so it uses them as graving tools to carry on the work 
of destruction. The sea, in turn, performs its share of the work 
around the margins of the land, using its shingle and sand as hammer 
and file to batter its cliffs and to grind its rocks. Thus we see that 
the forces at work fall naturally into three groups—first, the atmo¬ 
spheric agencies; second, running water (with which may be classed 
flowing ice, or glaciers); and third, the sea. Let me illustrate the 
action of each of these three by phenomena which have recently come 
under my own observation. 
Ten days ago a heavy fall of snow occurred, followed by frost, 
and then a sudden thaw. In walking along the footpath outside the 
Barracks wall, on the Dunkeld Road, I noticed that the snow at the 
foot of the wall was thickly sprinkled with sand, as if it had been 
shaken from a pepper-box, in a regular line. The wall is built of Old 
Red Sandstone, of a yellowish-grey colour and soft in texture, and on 
comparing the grains of sand on the snow there could be no doubt 
where they had come from. Here, then, was a striking example of 
the unaided work of frost in disintegrating a rock surface. The 
stone had first absorbed moisture from the atmosphere into its pores; 
when frost came the moisture froze, and in doing so expanded, and 
thus forced the component grains of sand slightly apart, though still 
holding them together in its ice-grip ; lastly the thaw came, melted 
the ice in the rock’s pores, and the loosened particles of sand fell on 
the snow-covered ground by their own weight. 
A still more instructive example of surface weathering was one 
I came across only to-day. At the corner of Carpenter Street and 
Cherry Lane is a house which, I am informed, is about ninety years 
old. It is built partly of porphyrite (“ whinstone ”) and partly of 
sandstone (“freestone”), the latter being laid on edge, and the two 
being mixed indiscriminately. The hard porphyrite, being scarcely 
worn at all, stands boldly out from the sandstone, and forms an 
index to the amount of weathering which the latter has undergone. 
On measuring the difference of level of the two, we find that the 
sandstone has been worn away in places to a depth of three inches. 
This shows the rate of weathering, in this instance, to have been one 
inch in thirty years. 
On mountain slopes, and other places where the rocks are not 
* Vide “ Theory of the Earth, 5 ' by Dr. Hutton, 1795 > a l so ** Illustrations of the 
Huttonian Theory of the Earth, 55 by John Playfair, 1802. 
