Mr. D. Sharpe on the Bala Limestojie. 527 



Sharpe identifies with the No. 2. of Mr. Bowman's lower division of 

 the Upper Silurians, the probable equivalents of the Wenlock shale. 

 Mr. Sharpe then describes the Bala series of rocks, beginning 

 with the uppermost beds. 



1 . Dark blue slate. — Worked at Craig Calettwr for good roofing- 

 slates and flags ; in one quarry the beds dip W.N.W 35°, and the 

 cleavage planes dip W.N.W 65° ; in another the beds dip W. 70° and 

 the cleavage W. 80°. Between Dinas Mowddy and Mallwyd it is 

 largely quarried for good slate and flags ; the beds dip S.E. or E.S.E. 

 about 30° ; the cleavage is perpendicular, and strikes S.S.W. The 

 lower beds pass into a soft argillaceous slate of no value. The whole 

 is not less than 300 or 400 feet in thickness. 



2. Upper Bala limestone. — A dark blue bed ten feet thick, accom- 

 panied by calcareous slates and soft brown shales, with many fossils, 

 among wbich are Orthis canalis and O. compressa, and several new 

 species. Mr. Edward Davis, who accompanied the author, discovered 

 this bed at Pen-y Dall Gwm, four miles south-east of Bala, dipping 

 W.^S. 70° : it is supposed to follow a line bearing N.N.E., much 

 broken up by faults*. 



3. Rotten argillaceous schist and indurated shale. — Light grey, 

 weathering to brown, with many joints and few fossils ; well exposed 

 in the valley of the Dwm-lach, above its junction with the Dyfi : 400 

 feet thick. 



4. Bala limestone. — A dark blue rock similar to No. 2, thirty or 

 forty feet thick, with calcareous shales and grits full of organic re- 

 mains, among which are Orthis pecten, anomala, vespertilio and bi- 

 lobata, Leptcena sericea, duplicata and depressa, and Spirifer radiatus. 

 This bed is much broken, and difficult to trace, but its general direc- 

 tion from Y-Garnedd, 1| mile east of Bala, to the upper valley of the 

 Cowarch, is nearly N.N.E. The line of limestone laid down, both 

 in Mr. Murchison's and Mr. Greenough's Maps, is compounded of 

 the beds No. 2. and No. 4. 



5. Grey slaty grits. — Occasionally streaked or passing into brown, 

 very hard ; well seen on both sides of the lake of Bala and in the 

 upper part of the valley of the Twrch ; usual dip E.S.E. 45°, but 

 much disturbed about the foot of the lake : the upper bed contains 

 Orthis canalis, anomala and vespertilio. In the lower part is a bed 

 thirty or forty feet thick of impure grey limestone with many frag- 

 ments of Trilobites and other organic remains, among which Mr. 

 Sharpe recognised Bumastus Barriensis, Trinucleus Caractaci, Illcenus 

 crassicauda, Orthoceras approximatum, and Lituites cornu-arietis. This 

 bed was only seen near Rhiwlas and Llan-y-ci, on the north-west of 

 Bala. The grits below the limestone are similar to those above, and 

 contain Orthis canalis and vespertilio, Leptcena sericea and Asaphus 

 tyrannus. The whole exceeds 500 feet in thickness. 



6. Rotten grey clay-slate, weathering to brown, forming the 

 moor between Bala and Arenig, and exposed where Cwm Croes joins 

 the valley of the Twrch : supposed to be 500 feet thick. 



* Mr. J. B. Morris has since met with the same bed in the valley of the 

 Dyfi at Blaen-y-Pennant. 



