176 ON THE REVOLUTIONS 



stands, composed partly of deposites, and partly of protected 

 strata, extends, gently sloping, for about a mile to the east- 

 ward, from the Castle to the Abbey of Holyroodhouse, where 

 the tail terminates. Round the western, southern, and north- 

 ern sides, a hollow valley occurs, which, towards the north, is 

 still a marsh, and was once a lake, being known by the name 

 of the North Loch. 



Corstorphine Hill, which, as seen from Edinburgh, occupies 

 the horizon to the north-west, affords, in one respect, an ex- 

 ample of the other case just mentioned. It consists of a ridge 

 of about a mile and a half in extent, rising in the middle, de- 

 clining gently at both ends, and pointing from north to south, 

 with a declination of about 20° to the east. It presents a 

 smooth face of whinstone to the west, towards which the mass 

 dips in parallelism with the strata beneath it. Upon its eastern 

 side a hollow valley occurs, in which the old castle of Craig- 

 crook stands, and from its southern extremity a tail extends to 

 the eastward, lying between Ravelstone and Murrayfield, upon 

 the southern face of which Murrayfield stands. 



Thus, the Castle of Edinburgh, gives an example of the ef- 

 fect of a narrow obstacle ; and this hill, of a broad one, in so 

 far as it has a valley on the side towards which the stream was 

 flowing. 



We have endeavoured, in the last communication to this So- 

 ciety, to account for the formation of such lakes as occur in 

 alpine and rocky districts. The circumstances just pointed 

 out, explain the formation of those which belong to districts 

 formed of frail and moveable substances. At Lochend a stri- 

 king example occurs, of a lake produced upon the upper side 

 of an obstacle, in consequence of local acceleration. 



Immediately on the east of Corstorphine Hill, a set of firm 

 jrocks, or little hills of sandstone occur, rising up from this hol- 

 low, 



