NATURAL HISTORY OF THE GENUS ASTACUS. 
213 
interior of the island. The elevation of the 
strata throughout Ireland is remarkably uni- 
form, being N. E. and S. W. in almost every 
part of the island ; to this remark, however, 
there are some exceptions, as, in the county 
of Tyrone, where the elevation of the strata 
is from N. to S.- 
From what has been stated, it is obvious 
that the primary rocks generally occur neac 
the coast, constituting the mountainous re- 
gions of Down, Donegal, Mayo, Galway, and 
Wicklow, &c. These regions containing all 
the usual primary masses ; as gneiss, mica- 
slate, clay-slate, and quartz rock, present in 
each locality many interesting appearances, 
which we have not sufficient leisure to detail. 
Quartz rOck, however, occurs at Dunmore 
Head, under some interesting modifications. 
It contains abundance of globular concentric 
concretions-, dilfering, in no respect, in their 
structure from the fibrous masses found in 
trap, and, like them, decomposing in crusts. 
In Donegal, beds of primary limestone 
occur, often alternated with mica slate, and 
have, in many cases, been changed into 
dolomite. 
Mr. G. then remarked that his informa- 
tion concerning the transition formations was 
less complete. These rocks consist of the 
greywacke and old red sandstone formations. 
In Cove of Cork both these are schists con- 
taining fossils. 
These transition and schistose rocks are 
succeeded by the mountain limestone, which 
occupies about two-thirds of the whole sur- 
face of Ireland. The organic remains found 
in these calcareous rocks are, in general, the 
same as those found in England. 
This limestome is succeeded by the coal 
formation. The newer secondary strata only 
occur in the north of Ireland, where we have 
the new red sandstone, with gypsum and 
beds of magnesian limestone, and these rocks 
are succeeded by lias, oolite, and chalk. 
2. Dr. west AFTERWARDS READ 
AN ABLE PAPER ON THE GEOGRA- 
PHY OF SOUTH GREENLAND. 
Tuesday j llth August. 
3. THE REV. ARCHDEACON VER- 
SCHOYLE READ AN INTERESTING 
PAPER ON A SERIES OF TRAP 
DYKES WHICH OCCUR IN THE 
COUNTIES OF MAYO AND SLIGO. 
These dykes are remarkable for the length 
and distinctness of their course. Their 
elevation is E. & W. They are, conse- 
quently, parallel to each other, and one of 
them has been traced for a distance of forty- 
five miles. These dykes, during this long 
course, intersect a great variety of rocks, as 
slate, sand-stone, limestone, mica, and slate, 
&c., and here produced many curious changes, 
converting the sandstone into quartz, and even 
giving it a columnar form. It was the opi- 
nion of the Reverend gentleman, that these 
veins, or a series of them, extended across 
the island. Mr. Griffith here remarked, 
that between Dundrum and Dundalk, on the 
opposite side of the island, a great number 
of trap veins had been observed. 
4. PROFESSOR JOHN PHILLIPS 
NEXT READ A PAPER ON THE FOS- 
SIL ASTACIDJE. 
The speaker commenced by making some 
remarks on the natural history of the genus 
Astacus. Of the species which compose the 
genus Astacus, as at present existing, some 
are found in salt and others in fresh water. 
There are two empiric characters by which 
the marine may be distinguished from the 
fluveatile Astaci : In the marine species 
the lateral divisions of the tail are trans- 
versely divided, while in the fresh water spe- 
cies the division is longitudinal. The marine 
species have also large didactylous claws to 
the first pair of feet. 
All the fossil species possess those charac- 
ters which belong to the marine division. 
Proceeding to investigate the question as to 
the possibility of identifying strata by means 
of their organic remains, it was remarked 
that the study of the fossil species of the 
present genus did not afford results very 
favourable to such a hypothesis. Confining 
our attention to the oolite and lias, it was 
observed that one species of Astacus was 
found in every bed, from the lowest of the 
lias to the uppermost of the oolite. 
One species was confined to the coral rag ; 
four species were peculiar to the green sand ; 
some of the species were more local, and 
others appear to have had a wider geogra- 
phical distribution, as is the case with the 
Astaci of the present day. 
MR. GRIFFITH THEN RESUMED THE 
EXPLANATION OF HIS GEOLOGI- 
CAL MAP, AND DESCRIBED THE 
ERUPTED ROCKS WHICH HAVE 
BEEN OBSERVED IN IRELAND. 
He divided the unstratified masses into 
three portions : 1. Those occurring in transi- 
tion and primary rocks : 2. In the older 
secondary : 3. In the newer secondary. 
It was remarked that the limestone which 
comes in contact with the erupted rocks of 
the primary division, is often changed into 
dolomite. 
Green stones occur among the older secon- 
dary rocks in the county of Limerick. These 
green stone beds are apparently interstrati- 
fied with the lime-stone, but fragments of 
this latter rock are included in the trap. 
The trap veins occurring in the ncAver 
secondary formations, as in the chalk of 
Antrim, are already sufficiently well known. 
Mr. G. is of opinion that the porphyry of 
Sandy rock is merely a modification of the 
ochre beds which are observed at the Giant’s 
Causeway, as there is a striking resemblance 
between the two rocks, in point of mineral 
character, and both contain nodules of meso- 
type. 
