On Cambro-Silurian and Silurian Rocks. 145 
series or highest part of the Cambro-Silurian may be seen in the 
following places, viz.: in the upland valley east of Slievenaman, 
in the Vale of Aherloe (Galtees), in Slieve-na-muck, in Slieve 
Phelim, in Slieve Arra, in the Cratloe Hills, in Slieve Bernagh, 
and perhaps in Slieve Aughta, and in West Mayo. 
Silwrvans.— In a former paper, certain rocks supposed to belong 
to this formation are discussed ; (On the Old Red Sandstone— 
so called—of Ireland, ante, page 135). The unquestionable 
Silurian strata in Ireland always lie with a strongly marked 
unconformability upon the Cambro-Silurians. 
The oldest rocks of Ireland in which fossils, regarded as indic- 
ative of Silurian age, have been found, are those of Ballycar, in 
the Cratloe Hills, county Clare, a few miles north of the city of 
Limerick. But as these rocks of Ballycar appear to be in and not 
on the associated Cambro-Silurians, it seems to me that the 
Ballycar rocks are really Cambro-Silurians, and that the just 
mentioned fossils of Silurian type occur therein only as a colony. 
The probability of this is increased by the occurrence of the 
inverse phenomenon elsewhere, to be mentioned presently. 
There are three hitherto recognised tracts of fossiliferous 
Silurians, namely—lIst, in the Dingle promontory, county Kerry; 
2nd, at the junction of Galway and Mayo; and 35rd, at Ballagh- 
aderreen in north-east Mayo. Of these the second is the largest, 
it is also the one where there is the greatest thickness of strata 
exposed. The rocks in this area are both peculiar and interesting, 
but as they are fully described in the “Manual of the Geology 
of Ireland,” I will not dwell upon them now. I must, however, 
mention here these two remarkable circumstances, namcly, that 
the characteristic fossil of the highest group (Salrock slates) is 
considered by that eminent Palzontologist, Mr. T. Davidson, to be 
of Upper Llandovery type ; while here, as also in the Dingle pro- 
montory is a zone carrying fossils of a Cambro-Silurian (Caradoc) 
type, having above and below it, especially in N.W. Galway, a 
considerable thickness of rocks in which are found fossils of Upper 
Llandovery types. 
The groups of fossils indicative of the age of the English Siluri- 
ans, are to some extent mixed up together in the Irish rocks ; on 
which account they are not in Ireland a reliable test of the age of 
the rocks. Still it would appear that the Irish Silurians probably 
