PROCEEDINGS OP THE PERTHSHIRE SOCIETY OP NATURAL SCIENCE. 
225 
Another low-level clay was turned up lately in County Place, 
one of the lowest-lying portions of the district, and in this 
evident track of a former channel of the river quite a deposit of 
fresh-water diatoms was expected. None, however, of any de¬ 
scription were found; but the Spongiolilhis , as usual in lower 
depths, appeared in small numbers. 
The next higher in point of position that was examined was 
taken from the street in York Place, opposite to the Infirmary, 
during some repairs to pipes. It was of an ochry yellow colour, 
and abounded with fresh-water forms of diatoms, among which 
some beautiful large specimens of Surirella plicata were found, 
some of Gomphonema and Nitzschia, and also several brackish- 
water forms of Pleurosigma and Navicula. Several Melosirce, 
Nitzschice, and Pinnularice were also found, as well as a 
beautiful marine diatom of a rich brown colour, shaped 
like a round bead, and covered with delicate and intricate 
markings, too minute to be seen except with very high powers, 
and not well seen with them on account of the globular shape 
throwing all but a small portion out of focus. What corresponds 
to the perforation of the bead is found with the high powers to 
be a delicate membrane, also finely marked. I have not ascer¬ 
tained the name of this diatom. 
The next clay in order of height was at the site of St Cathe¬ 
rine’s Chapel, near the top of the Old High Street, where at a 
depth of 12 feet a dark blue clay was found, containing quite a 
number of fresh-water diatoms, among which were some magni¬ 
ficent specimens of Eunotia and Pinnularia, and a great many 
of Melosira , some of them single and others connected in rods. 
Among them was afinebraekish-water Zygoceros, a large square¬ 
shaped diatom with projecting corners, the body being a high 
rounded disc with two transverse bands and covered with very 
numerous minute markings, only to be seen with a high power. 
Some large specimens of Nitzschia were also found, and sponge 
spicules of various forms. 
Above this were beds of yellow and blue clays intermixed, 
and in these, especially the blue, diatoms were very numerous, 
especially marine andbrackish-water ones. Conspicuous among 
these were the Pleurosigmce, some very large and some very 
small, but all very delicate, and difficult to pick out and mount. 
Many diatoms of the same sigmoid shape, but without the central 
nodule and raphe, and prettily marked along the edges, were 
found of all sizes, but invariably long and narrow. Another 
marine diatom, Isthmia inervis, was found here; and large 
numbers of a very common but very pretty brackish-water one, 
Nitzschia constricta. Another common brackish-water form 
is very plentiful here, but I have not been able to ascertain 
the name, and I have not found it elsewhere in Perth. It 
much resembles Surirella Caledonica. It is very beauti¬ 
fully marked with minute dots converging towards the centre, 
and is of an ovoid form, with pointed ends—one surface 
being flat and the other convex. There are also quite a number 
of Actinoptychus, a fiat circular diatom, the surface of which is 
divided by six raised ridges into six compartments, the three 
alternate ones of which are on the same level and finely spotted. 
Perhaps the most common of all are two or three forms of 
Coscinodiscus. One of these has its minute hexagonal markings 
arranged somewhat like the engine-turned pattern on a watch- 
case ; another has its somewhat larger markings placed 
without any definite arrangement, while a third has them 
arranged in a series of curves or segments of circles, the concave 
sides directed towards the circumference. There are also con¬ 
siderable numbers of the brown bead-like diatom formerly 
mentioned, all of these being brackish-water forms. Our old 
friend the Spongiolithis is also very common, but no fresh-water 
forms are to be seen in the upper beds of this clay. 
The next higher clay examined was that in the brickyard 
which now forms the site of the New Auction Mart, but here 
only one diatom was fouud—a marine one—besides some sponge 
spicules. In the highest clay of all, taken from behind the Poor- 
house, 2 feet from the surface, only a few small diatoms were 
met with, and some broken fragments of spoDge spicules. 
In none of the clay beds examined were any calcareous shells 
found, with the exception of two Foraminifera found at St 
Catherine’s. These, on account of their buoyancy, were both 
lost in attempting to mount them in balsam. 
It will be evident from a consideration of the foregoing, that 
the basin in which the City of Perth is built, has for an immense 
period of time, comparatively speaking, been in most respects 
similar to one of the West Highland marine lochs, differing 
from them chiefly in having a very narrow entrance towards the 
sea, and in having a very large river flowing into it, rendering 
its waters to a great extent brackish. It is also evident that the 
red glacial clay, without aDy remains of organic life, has been 
the original bottom of that loch, which has gradually silted up 
by deposition of the mud brought down by the river, till 
nearly forty feet of clay was deposited over its bed, during 
which time countless millions of minute organisms which can 
only exist in brackish water lived and died during thousands of 
years—for in thousands of years only could such a depth of mud 
have been deposited over such a large area, These organisms have 
left silicious skeletons of the most beautiful construction and 
most elegant and intricate ornamentation to tell of the marvellous 
power of their Creator, who could dispose of so much grace and 
regularity of ornament and design where it cannot be seen with 
the naked eye, but can in many instances be revealed only by 
the very highest powers of the microscope. 
It appears evident also that the level of the land of this 
country had rapidly changed, in two successive upheavals of 
about 25 feet each, and that from the deep bed of clay that had 
thus filled up the Perth basin, the river had, by successive 
changes of its position, gradually worn away the major portion 
of this clay deposit, till, having attained its present level at 
about the level of the sea beyond, it can now get no lower 
unless the land again rises, and gives it the power to cut out a 
deeper channel. 
On reference to the Ordnance Map, I observe that the lower 
plain on which the larger portion of the city of Perth is situated 
has an average height of 17 feet above sea-level. The higher 
plain or haugh along the eastern and northern sides of the Perih 
Basin, in which the diatoms are of marine or brackish origin, 
averages 35 feet, and the various sandy beaches outside of this 
about 45 feet. 
