12 



The first section begins westward, at the igneous chain of Arenig 

 Mawr, the natural boundary to the district ; it crosses the town of 

 Bala, and ends eastward at the Calettwr, where a dark slate, the 

 upper bed of the Bala series, abuts unconformably against the clay- 

 slate of Moel-lialog, which is referred to the Cambrian system. This 

 section places the Bala beds in a detached trough, and shows that 

 they do not dip under the Berwyns : but their succession is not well 

 shown, owing to the disturbed state of the surface. 



The other section is in two parts ; from the head of the lake of 

 Bala up the Twrch to Bwlch y Groes, and across the Dyfi by Dinas 

 Mowddy and Mallwyd, which line the author recommends to 

 those who wish to study this series, as the rocks are well exposed in 

 the upper part of the valleys of the Twrch and Dyfi : on the west it 

 begins at the northern prolongation of the igneous chain of Arran 

 Mowddy, and continues eastward through a conformable succession 

 of beds up to the Upper Silurian ; each section shows the whole of 

 the Bala series, the upper bed of blue slate, which on the Calettwr 

 rests unconformably against the Cambrian clay-slate, being the same 

 which is overlaid conformably beyond Mallwyd by an Upper Silurian 

 series of soft blue or liver-coloured shales alternating with hard, grey 

 grits, without cleavage or fossils, dipjoing east-south-east, which Mr. 

 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 which are OrtJiis 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 ii- 



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

 Dyfi at Blaen-y-Pennant. 



