LLANDEILO FLAGS (RANGE AND DISLOCATIONS OF).

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on the high road from Caermarthen to Swansea , where pebbles of white quartz of the size of smallbeans are disseminated in a base of dark grey schist, with some felspar and green earth, the stratabeing vertical, and their strike from north-east to south-west. It is no easy task to distinguishthese sandstones from the overlying and underlying formations in their progress westward from theright bank of the Towy to St. Clears, on account of great denudations, and the apparent absenceof fossils and calcareous strata; but before we enter Pembrokeshire, the Llandeilo flags reappearingin great force, are again clearly separated from the Upper Silurian Rocks by thick masses of yellowsandstone, largely quarried at Llandwror, and containing fossils of the true Caradoc formation.(See Map.)

“ Llandeilo Flags.”

Llandeilo flags, distinguished by the presence of the Asaphus Buck'd and A. tyrannus,and underlying the great mass of Caradoc sandstone, are exhibited on the left banks ofthe rivers Sowdde and Towy below Llangadock. They extend thence by Tan-yr-alt,Pen-y-banc and Pompren-arreth, to the low hills of Pentref and Tir-wyn-fach, wherethey strike across the Towy, occurring in great force at Llandeilo and in DynevorPark. From Llandeilo to Caermarthen , this formation is seen on the right bankof the river, chiefly in detached and broken masses, near Llangathen and Llanegwad,the only places where I have detected the rock on the left bank being at Golden Groveand at Capel dewi. Beyond the latter point the calcareous matter thins out, and doesnot reappear till we reach Clog-y-frain, on the borders of Pembrokeshire, though thecourse of the formation is traceable at intervals by its organic remains, as at Pensarn,&c., near Caermarthen .

By consulting the map, it will be seen that between Llangadock and Caermarthen , aspace about fifteen miles in length, and from half a mile to two miles in width, theseLlandeilo flags have been singularly thrown about with divergent strikes and reverseddips. At their north-eastern end a transverse section (PI. 34, fig. 5.) from Tan-yr-alt,to Blaen-dyffrin-garn, passes over the low hills of Pen-coed, Pen-llan, and Tyr-y-garn,exposing beds of black calcareous flagstone, occasionally very pyritous, and more orless charged with trilobites, alternating with thickish bedded strata of grey, quartzosesandstone and dark shelly grits 1 . These flagstones have a prevailing direction to thesouth-west, and dip in opposite directions, i. e. both north-west and south-east, at highangles. In following the strata to the south-west, they are subjected to several breaks,by one of which they are deflected to the north-north-west, or nearly at right angles totheir prevailing strike ; but they resume their south-westerly direction, and range withtolerable regularity along the western flanks of Cairn-goch and Carreg-cegin, plungingat one or two spots at high angles beneath the overlying sandstone. (PL 34, fig. 6.)At Pompren-arreth, the subordinate beds of grit thinning out, the blackflags and shale

Encrusting springs issue from some of the more calcareous beds.