W. T. Black — Grooved Boulders, Edinburgh. 321 



the lip of the cataract without support. In this way great masses 

 of the limestone are constantly breaking off along lines of jointing, 

 and the Falls are gradually receding up the river, and the length 

 of the ravine below the cataracts is constantly being increased. If 

 the strata through which the river cuts were quite horizontal, there 

 would be no limit to the recession of the Falls and to the length of 

 the gorge which might be formed in this way. But, the strata have 

 a slight dip in the opposite direction to that in which the river flows, 

 or in the same direction as that in which the Falls are receding. It 

 results from this that when the river has cut its way back for a 

 certain distance, the conditions of the case must change. The hard 

 and compact limestone which now forms the edge of the Falls will 

 ultimately come to form the bed of the river ; and as the strata 

 which succeed this upwards are of a softer nature, the place of a 

 single fall with a long ravine below it will finally be taken by a 

 continuous rapid or a succession of minor cascades. As has been 

 jDointed out by Hall and Lyell, this is precisely the action which is 

 going on at the Falls of Niagara, but upon a gigantic scale. I have 

 endeavoured to show, however, that veiy similar phenomena may be 

 produced under very different conditions ; as is seen in Watkins 

 Glen, where the strata through which the stream cuts are of a very 

 uniform nature, but are regularly intersected by two sets of vertical 

 joints cutting one another at right angles. 



VII. — Grooved Boulders in Diluvial Clay, Watson's Hospital, 



Edinburgh. 



By "W. T. Black, Esq. 



THE excavations for the foundations of the new Infirmary in 

 the grounds of Watson's Hospital, Edinburgh, have disclosed 

 several Boulders, which appear lying in the Diluvial-clay. This is 

 a rdarly clay, and has been dug to the depth of about five feet, in 

 several horizontal terraces, down the slope from north to south. 

 These Boulders were of sufficient size to be left behind in situ by 

 the labourers, and about six large ones were found in the north and 

 twelve in the south area of the grounds. They were of trappean 

 rock chiefly, being rounded on the angles and sides, and of a dark 

 green crystalline structure internally, A large one in the north area 

 was of a metamorphic granitic character ; one in the same area of a 

 buff-coloured sandstone ; and another in the south was composed of 

 the same rock, and grooved also. 



The grounds are in the form of an unequal quadrilateral figure, of 

 the mean length of 850 feet and breadth of 750 feet, and are divided 

 into two areas north and south by the buildings of the hospital. 



The slope of the grounds of Watson's Hospital is towards the 

 south, down to the Meadows, from Lauriston Street, on the north ; 

 and the angle may be about 2|^°, or 1 foot in 20. In the northern 

 area, the dimensions of the larger granitoid Boulder (No. 2) were 

 1ft. Sin. high, 2ft. 4in. long, and its horizontal circumference was 

 7ft. 6in., and the depth of the platform it was found on was 4ft. Sin. 



VOL. IX. — NO. xcvii. 21 



