PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. Cxliil 
notice that the sub-soil is tinged a slight brown colour for a distance 
of nearly three feet below the surface soil. 
2. For our next section we will go to a part of the county where 
the geological conditions are entirely different, namely, the volcanic 
region of the Sidlaws. Those of you who attended the excursion to 
Pitroddie last summer will remember the small quarry at the top 
of the Den which we visited before turning homewards. At the 
entrance to this quarry we came upon an instructive section, to 
which I made a passing reference in my address in November last. 
Here a bed of tuff, or volcanic ash, had been formed in a hollow of 
the porphyrite, or ancient lava bed, the deepest part being about 
four feet in thickness. On the top of the whole rested a compact 
bed of boulder clay about three feet in thickness. This tuff generally 
forms a fairly compact rock, and so it appeared to be here, where it 
had been cut through to form the entrance to the quarry; but when 
we applied our hammers to it the whole proved to be a mass of 
loose debris as friable as when it was ejected from the mouth of the 
volcano. As this consisted of the pounded fragments of original 
volcanic rocks, rich in silicates of soda, lime, potash, and magnesia, 
as well as protoxide of iron, it will readily be understood why the 
flora of the Sidlaw Dens is so rich and varied. The reason of its 
present condition at this particular spot was of course that, after it 
had been broken up by the disintegrating forces already referred to, 
the covering of stiff boulder clay had protected it from the denuding 
action of surface agencies. In its present state it is a storehouse to 
replenish exhausted soils as soon as the boulder clay shall be re¬ 
moved from above it. 
3. For our third section we choose a locality in the Old Red 
Sandstone district, where we shall again find a corresponding differ¬ 
ence in the soil conditions. The precise spot I have selected is one 
which is now familiar to many of you, namely, the Burghmuir 
Quarry, where the ancient stone cist was discovered some weeks 
ago. The geological lessons of this section have been impressed on 
my mind through an error which I fell into on the occasion of my 
first visit. Noticing that the cist rested on what appeared to be a 
bed of rude masonry, consisting of successive layers of thin slabs 
of sandstone resting loosely on each other, and being on the outlook 
for traces of human workmanship, I concluded that this was a 
foundation built up from the solid rock for the cist to rest upon. 
When Mr. Armstrong Hall pointed out, however, that the same 
structure occurred at other parts of the quarry, it was at once 
apparent that this represented the “ brash,” or intermediate stage in 
the natural disintegration of the sandstone back into its original 
sand. The disintegrating forces first attack the sandstone along the 
planes of jointing and of stratification, thus producing the appearance 
described. I have specially referred to this section and its associa¬ 
tion with the antiquarian find on account of the bearing which it has 
on a very interesting geological problem, namely, the rate at which 
disintegration proceeds. Obviously this is a question on which 
geologists have very scant data to work, and everything which seems 
to throw any light on it is worth noting. Now, the slabs of which 
