542
 

SILUEIA.
 

E.—Astacodermata.
 

Mr. Henry Woodward has suppUed the following note on the pecuHar little
fossils referred to at pages 133 & 134:—

With the exception of Astacoderma serratum (which is perhaps a fragment of
the margin of a carapace or of a body-segment of Ceratiocaris), and of A. spi¬
nosum (not clearly referable to Ceratiocaris), the several forms of Astacoderma
described and iigured by Dr. Harley are doubtless founded on the teeth of
PhyUopod Crustacea, such as Ceratiocaris. For examples of such teeth in
Dictyocaris, an allied genus, see the ^ Geological Magazine,' vol. ii. p. 401, pi. 11.

G.—Old Red Sandstone of Forfarshire (p. 250).

In treating of the classification and palaeontology of the Old Red Sandstone
of Forfarshire and other parts of Scotland at p. "250, aUusion to the researches of
the Rev. Hugh MitcheU was inadvertently omitted. In the Quart. Joum. Geol.
Soc. vol. xvii. p. 145 et seq., Mr. MitcheU describes the position of the beds of the
Old Red Sandstone in the counties of Forfar and Kincardine, and at p. 146 he
classifies the fossils that have been found there and elsewhere; and amongst
them he enumerates the small spores known as Pachytheca, which are not rare
in the uppermost Silurian beds at Ludlow in Shropshire.

In addition to the references made at p. 251 &c. to Mr. Powrie's discoveries,
it is right to refer the reader to the publications of this author on the Old Red
Sandstone and its fossils, in the Quart, Journ. Geol. Soc. vol. xvii. p. 534 et seq.,
and vol. xx. p. 413 et seq.
 

H.—Caithness Flagstones of the Old Red Sandstone (p. 258).

The Flagstones of Caithness, which were first described by me in the year
1827, under the name of' Bituminous schists' (Trans. Geol. Soc. 2nd ser. vol. ii.
p. 213), are in many places impregnated with bitumen, chiefiy resulting from
the vast quantity of fishes imbedded in them. Their most durable and best qua¬
lities as flagstones are derived from an admixture of this bitumen with finely
laminated siliceous, calcareous, and argillaceous particles, the whole forming a
natural cement more impervious to moisture than any stone with which I am
acquainted.

The following analyses of several specimens of Caithness flagstone and the
accompanying bituminous shale, from the celebrated quarries of Castle Hill,
belonging to my friend Mr. G. Traill, M.P., are indeed of high value, having been
prepared under the superintendence of that distinguished chemist Dr. Hofmann.

These results completely sustain the opinion I was led to form upon the spot,
that the peculiar tenacity and durabUity of these flagstones is due to the manner
in which the silica and alumina are cemented together by certain proportions of
calcareous and bituminous (organic) matter.
 

Mineral analyzed.
 

Silica and
Silicates
insoluble
in H CI.
 

Oxide of
Iron and
Alumina.
 

Carbonate
of Lime.
 

Organic
matter.
 

Water,
loss at
100° C.
 

Salts of
Magnesia,

the Al¬
kalies, &c.
 

Total.
 

No. 16. Top flag .
No. 7. Middle flag
Bituminous Shale.
No. 1. Bottom flag
 

68-40
69-45
69-96
61-39
 

10-21

11-50

8-15

4-87
 

10-93
10-66

7-72
21-91
 

3-88

5-79

10-73

3-40
 

0-42
0-40
0-53
0-20
 

6-16
2-20
2-91

8-23
 

10000
100-00
10000
100-00