22S Proceedings of the Royal Irish Academy. 
the valley of TJdias, In both sections the corresponding beds are 
marked with the same numbers. The bed marked JSTo. 7, which im- 
mediately overlies the dolomite, and caps the heights at both sides of 
the valley of Udias, is a fossiliferous limestone, which towards the sea 
is covered by soft greenish sandstones. The great dolomite bed 
which is marked No. 8 crops out with physical characters so marked, 
and so distinct from any of the overlying beds, that no indications of a 
passage from the one to the other are recognizable. The extremely 
fissured state of this dolomite, and the many jagged points which are 
visible wherever it shows itself, are, we think, strong evidence that 
the upper sm^face of the dolomite is a surface of denudation upon 
which the overlying limestone was deposited. This limestone con- 
sists of thin, compact, closely-bedded, often shaly, marly or sandy beds, 
which present a marked contrast with the limestones underlying the 
dolomite, and marked 9, 11, and 12. The bed marked 10 is a shaly 
limestone, which yields at another point a very peculiar cement of good 
quality. The group of beds marked 13 consists of red shales, sand- 
stones, and limestones, which crop out again in the valley of Cabezon 
de la Sal, to the south of Udias, as also on both flanks of the Dobra 
chain, still further south. 
An important physical distinction between the rocks underlying 
and overlying the dolomite is furnished by the fact, that the joints 
which fissure both the dolomite and the underlying rocks do not 
seem to have extended themselves upwards into the overlying rocks, 
which, in this district, do not anywhere afford any deposits of ores ; 
while the underlying beds abound in them. This discontinuity of 
jointing vertically is most marked, and tends to confirm the opinion 
that the dolomite closes the Jurassic formation in this district, and that 
there is a gap of some extent between the dolomite and the cretaceous 
overlying limestone. 
Our next task was to determine the stratigraphical position of the 
overlying cretaceous rocks. From the study of the limited number 
of fossils at our disposal, as well as from stratigraphical and 
lithological considerations, we assumed the existence of three stages 
of cretaceous rocks, the turonian, the cenomanian, and the neo- 
comian. The limestone overlying the dolomite we looked upon as 
belonging to the latter. M. D'Archiac concluded from the obser- 
vations of M. de Yerneuil that the neocomian stage is wanting, 
so that M. de Yerneuil' s third stage — for he also recognizes three — • 
would be the cenomanian. 
The age of rocks can only be determined with certainty by paleon- 
tological evidence. In this district the rocks that form the base of 
the cretaceous and the top of the jurassic formations are, unfortunately, 
in such places very poor in fossils, and of those that do occur we had 
only very few when writing our paper. We had, however, no doubt 
of the jurassic character of the dolomite — as to this the evidence was 
satisfactory. On the other hand, we considered our classification of 
