212 
Proceedings of the Royal Irish Academy. 
and 300 — the dispersion of the Dublin Society's spectroscope being 
about eighty. Mr. Burton has undertaken to make the object mirror, 
and Mr. Grubb will make the prism, the window of the tank, and all 
the rest of the apparatus. 
An instrument constructed in the same way, but with its semi- 
prism built up of wedges of flint glass, cut out of the disks manufac- 
tured for object lenses, with castor oil between them, would appear to 
offer great advantages as a spectroscope for making accurate measures. 
Tor the observer could at once tell which line is in minimum devia- 
tion by its coincidence in the field of view with the middle of the 
vacant patch ; and the position of the prism, which is the only move- 
able part of the apparatus could be read off with the precision of an 
astronomical observation. 
XXXIII. — Discovery of Tish-eemains m the Alluvial Clay of the 
River Foyle, with Observations on the Existence and Disap- 
pearance OF AN Upper Lough Foyle, and on the former Insulation 
of Derry and of Inishowen. By G. Sigeeson, M. D., Ch. M., E. L. S. 
[Read June 26, 1871.] 
The discovery of organic remains in alluvial deposits, whilst generally 
of value, is in the present case of peculiar interest, as their apparent 
absence from such deposits in the locality has attracted comment. In 
the geological portion of the Ordnance Survey of the adjoining county 
of Londonderry, Captain Portlock thus wrote: — ''As yet no shells, 
either fresh- water or marine, have been found in the detritic gravel or 
clay of this parish [of Templemore]. In other parts of Ireland, marl, 
abounding in fresh- water shells, is frequently the substratum of the 
smaller bogs, establishing fully their former lacustrine state, which is 
further supported by the occasional discovery of ancient canoes within 
them. Under the larger bogs, clay or gravel, without shells, is more 
commonly found ; but before the full bearing of this deficiency on the 
question of formation can be estimated, the bottoms of existing lakes 
should be carefully examined in all positions, and under all circum- 
stances, and the abundance, scarcity, or total absence of shells in the 
shingle of the present sea or lake shores, carefully ascertained at various 
points and under various conditions." Then, referring to the valley 
that bounds upon the west the hill on which the fortified city of 
Derry is situated, he remarks : — '' Indeed, as regards this immediate 
parish, the insulated valley of Derry, or Mary Blue's Burn, seems a 
stepping stone in the inquiry, as it can be nearly proved from history 
to have been a channel of the river ; and yet it still exhibits a bottom 
of gravel and clay without shells — a circumstance in some measure to 
* The beam of light in passing through the prisms of this spectroscope is higher 
than it is broad ; so that if estimated by the height, the standard power would be 
nine, and the dispersion 108 ; if estimated by the breadth of the pencil, the stand- 
ard power would be five, and the dispersion, 60. The instrument, therefore, 
should be credited with a dispersion which lies somewhere between 60 and 108. 
