SILURIAN STRATA.

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in Mr. Griffith’s first Map , from the old red sandstone. Thediscovery by our collectors of the rich mine of fossils they con-tain, has, however, now placed them in their true position ; andthough other Silurian districts in the south and west have sincebeen either discovered or more fully identified, the Pomeroy andDesertcreat district still stands pre-eminent amongst them. Theposition of these strata is remarkable (PI. G, fig. 2) : they restat their base on granitiform and hornblendic rocks, which rise upbehind them either in rough round knolls, like Bardahessiagh, orin sharper hills like Craigs, according to their composition. Theserocks are extremely varied and beautiful, and will be described indetail in Chapter XI. ; they are peculiarly interesting from themanner in which those of a syenitic or granitiform character areconnected with others of a greenstone type, and from the manyglimpses they afford, in their varied composition, of metamorphieaction. Taking this latter circumstance into consideration, itcannot be said that the actual base of the once stratified rockshas been here arrived at, although it is remarkable that the schistsnow remaining, excepting in their induration, do not exhibit anypeculiar marks of igneous modification, whilst the overlying sand-stone in the development of porphyries within its range, affordsnumerous and striking proofs of metamorphie action. The tractin Tyrone, which can with certainty be considered Silurian , is ex-tremely small, occupying a nearly triangular space, in the parishesof Pomeroy and Desertcreat, the base of which, running in theline of strike from E. by S. to W. by N., along the syenitic range,is about three miles long; and the perpendicular in the line of dip,southward, is also about three miles ; hence, as the dip, whereobservable, is upwards of 30°, the total thickness of the strata,if considered continuous, would be at least 7920 feet; but such isprobably not their condition, the strata having evidently sufferedmuch local disturbance, appearing at one point to stand nearlyupon their edges. The mass which exhibits the greatest appear-ance of continuity is that immediately south of the syenitic hillof Bardahessiagh; it extends in the line of dip about half a mile,and if estimated by the amount of dip, would therefore be 1320feet thick. The two other topographical spaces in which thestrata are exposed, are each less in extent, hence the thickness ofthe whole, supposing such a system of calculation correct, wouldbe about 3500 feet. One remarkable peculiarity in this deposit