The following measurements and data will furnish better than words the capacity and magnitude of the various reservoirs :

Or a total of 17,360 million gallons forming a superficial area of water, when all the reservoirs are completely full, covering 1,499 acres.

The following are the proposed lengths of the dams :-On the Elan river: Caban Côch, 600 feet; Pen-y-Gareg, 525 feet; Craig-Gôch, 625 feet. On the Claerwen river: Dol-y-Mynach, 938 feet; Cil-Oerwynt, 1,052 feet; Pant-y-Beddau, 720 feet.

The party, having by this time grasped some idea of the design, after a grateful vote of thanks to Mr. Maddocks, assembled to make a closer inspection of the tunnel. At the dinner hour, between one and two p.m., reports of blasting operations were heard, which are also conducted at the breakfast hour, between eight and nine a.m., and after the men have finished their day's labour; so as neither to cause any interruption at irregular hours, nor to endanger the safety of the workmen and visitors.

Directions and cautions to miners as to the use of Noble's gelignite, gelatine dynamite, dynamite, and other nitro-glycerine explosives employed for blasting, are conspicuously posted at the entrance to the Tunnel. Manual labour, occasionally assisted by the steam-drill, is chiefly depended upon in drilling the holes for the charges, wooden tools being used for the tamping.

So soon as the inspection of the Tunnel had been completed, the company assembled on the heights overlooking the excavation of the Careg-Ddu Dam, and the President made some remarks on the Physiography of the country.

THE PHYSIOGRAPHY OF THE DISTRICT.
BY H. CECIL MOORE.

The district we have passed over to-day is as interesting from its geological features as it is notable for the magnificent catchment area in which we are now assembled. The first half of our journey to day was over that system so familiar to us as the Old Red Sandstone of Herefordshire, and whilst we have been descending in the geological series of formation we have been ascending

from Hereford 189 feet 3 inches above sea level (as marked on the jamb of our Market Hall), to this elevation of 770 feet, where we now stand at the mouth of the aqueduct for the Birmingham supply. Those who were looking out of their carriages would have observed the transition from the Old Red Sandstone about a mile after leaving the Railway Station at Erwood. The character of the soil is conspicuously different and reminds us of the Upper Silurian soil in the district of Woolhope. The line of demarcation is well defined. About half way between Boughrood and Erwood Railway Stations the line crosses the small tributary Bachhowey, which separates the Upper from the Lower Ludlow rocks; if the course of the Backhowey be followed for about two miles the visitor will see remarkable contortions of these rocks at the waterfalls of Craig-y-pwll-ddu (the crag of the black pool). Travelling northwards from Erwood, just before reaching Aberedw, a fantastic grouping of gigantic rocks upon the right hand presents a remarkable resemblance to the masonry circumvallation tier upon tier of a fortified hill, like a miniature Gibraltar, and the horizontal stratification of the Upper Ludlow tilestones, due to disintegration of the softer parts from weathering, forms a marked feature. Near Mynnd Aberedw the Aymestrey Limestone takes up its normal position of Middle Ludlow. Near Builth we come upon a protrusion of igneous rocks, consisting successively of Felspathic trap, Felspathic ash, and Greenstone; Wellfield, two miles north of Builth, being on a narrow bed of Llandilo flags of the Caradoc or Bala series of the Lower Silurian between two rocks of Greenstone east and west. The river thence for four or five miles crosses over calcareous shale with rubbly concretional structure, representing the upper flags and shales of the Wenlock series, through six miles of which stratum the river Ithon flows by Disserth, before emptying into the Wye, upon its left bank, five miles north-west of Builth. One mile of Denbighshire flags and shales of the Upper Silurian are crossed at Newbridge, whence for the remaining seven miles to Rhayader the river Wye traverses sandstones and conglomerates of the Lower Silurian. Surely, within one day's march, a sufficient variety to satisfy the most grasping geologist.

The iron and sulphur and saline waters of Builth and Llandrindod owe their qualities to the iron pyrites and other mineral constituents in the erupted volcanic masses in their neighbourhood.

For a record of fossils found in the district of Builth see Transactions, 1865, pp. 133, 134, being Part 6 of the earlier issues of our Transactions in pamphlet form.

We have now brought our observations of the crust of the earth upon which we have to-day travelled up to the vicinity of Rhayader. We will now follow a section of ten miles in length bisecting the watershed of the rivers Elan and Claerwen in a direction across the basin running from south-east to northwest. Commencing three miles south-east of the dam at Caban Côch, following the argillaceous slates with beds of thin grit, we find at Cefn-y-gamrhiw, through Craig-y-cnwch and Gro-fawr, thick bedded sandstones and conglomerates remarkable for their occasional sudden development, the slates passing rapidly into sandstone and coarse conglomerates, with a few calcareous concretions and