SILURIAN STRATA.

475

viction is at once realized that they represent co-existing Fauna;. Theparticular position of the larger portion of the strata in the Silurian formation seems also by the same comparison to be low down, andI hare therefore considered two-thirds at least to belong to theLlandeilo flags and Caradoc sandstones, but, in ascending, the upperportion is found to have acquired a decided grit-like character, andto vary in the proportion of its fossils ; Trilobites are very rare andreduced to fragments, such as a portion of the cephalothorax ofTrinucleus, but Leptoena sericea still continues and is associated witha costated Spirifer, S. cyrteena, and a group is therefore formed ofa very different aspect to that of the lower schists; here, then, isprobably a higher member of the formation, Plate G, fig. 2; though,as yet, a transition by fossils, through the Devonian system to theCarboniferous , has not been here ascertained. Enough has been nowsaid to prove, that a true Silurian deposit, of a considerable verticalthickness, but of small lateral extent, unless a portion of the adjacentgrits belongs to it, exists in the County of Tyrone and again appearsin Fermanagh : * a few also of the Waterford fossils, contributed byMajor Austin and Captain James, have been described, and theyare equally conclusive as to the existence of a Silurian formationthere, which, however, has yet to be minutely examined and illus-trated. Strata, in Kerry, supposed similar, require also to be closelystudied; and, finally, Mr. Griffith has discovered Graptolitic bedsin the County of Meath, near Slane. f These are, probably, onlyscattered remnants of a formation once spread over larger spaces, butlike the mica schist below, and the old red. sandstone above, worn awayby the destroying agencies of nature: and if so, they supply in theirisolated condition stronger proofs of successive epochs of disturbanceand of rest, of destruction and of formation, than are even found in thevarying aspects of successive Fauna;.

In the Section Plate Gr, fig. 2, the following variations in the stratahave been noticed. At the base they are quartzose, and near theirjunction with the sienitic rocks massive; in one instance appearingto have become a hard, slightly calcareous quartz roclc. The principalniass is schistose, or flaggy, becoming near the base at Bardahessiaghgritty, and the large Orthis is there most abundant. In one localitythe slates are even and smooth, and particularly rich in Graptolites,—coinciding nearly in direction to the Graptolitic bed, and, probably,

‘Fragments of Trilobites, too imperfect for satisfactory description, occur in the Lisbellawschists; they are not, however, carboniferous.

T Nov. 5 , 1842, J! r _ Griffith informs me by letter that he has just discovered anothersmall Silurian distinct, connected with red sandstone, in the Comity of Mayo; thath differs from tile preceding, the slates alternating with impure calcareous beds, which aree d with upper Silurian fossils, but as yet no Trilobites have been found in them.