VOL. XLVr.J PHILOSOPHICAL TRANSACTIONS. 667 



away their whole substance. In the same manner, and by the same means, 

 were the stony particles replaced into those very cavities which the shells for- 

 merly filled; consequently these bodies were moulded exactly to the said 

 cavities. 



This remark carries a conclusion with it, that the hollows and solid parts of 

 these stones exactly answer to the hollows and solid parts of the very shells them- 

 selves ; which, had they been moulded in the very shells, must have happened 

 directly contrary ; the solid parts of the shells forming hollows in the stone, and 

 vice versa. In all sandy or lax earthy matter, fossil shells are very seldom found, 

 but only the moulded stones ; the loose texture of those substances giving free 

 access to water, vapours, and mineral exhalations, &c. which entirely corrode 

 and destroy the shells buried in it. 



The Stale of the Tides in Orkney. By Mr. M. Mackenzie* N° 492, p. 149. 



There is little or nothing uncommon in the manner of these tides. 



Ordinary spring tides rise 8 feet perpendicular, ordinary neap tides 3-^ ; ex- 

 traordinary high spring tides rise 14 feet; extraordinary low, only 5; extraordi- 

 nary high neap tides rise above 6 feet ; extraordinary small neap tides not above 

 1. Low water neap tide, at a mean, is about 3 feet above low water spring tide, 

 and high water spring tide about 3 feet above high water neap tide. 



On the coast of Orkney, and fair isle of Shetland, the body of the flood 

 comes from the north-west ; on the east and west coasts of Lewis, one of the 

 western isles of Scotland, it comes from the south. A league or two off the 

 coast, the strength of the stream is scarcely sensible, except when it is confined by 

 land, or near rocks or shoals. When the tide begins to rise or fall on the shore, 

 about that same time the stream near the shore begins to turn or reverse its 

 direction, a few irregularities excepted. 



The stream of tide changes its direction sooner near land than at a distance 

 from it ; insomuch that, in a place 2 or 3 miles from land, the turning of the 

 tide is 2 hours, or more, later than on the adjacent shore : at intermediate dis- 

 tances the streams turn at intermediate times. Hence a vessel may find a fa- 

 vourable tide near land, while it would be against her a mile or two from it ; and 

 the contrary. 



During the continuance of flood, the stream varies its direction gradually 

 from the east towards the south, and the stream of ebb from the west towards 

 the north : that is, if the stream, when it becomes first sensible, runs east, at 

 the latter end of the tide it will run south, if the proximity of land or shoals 

 does not hinder this change of direction. 



* Author of a treatise on Maritime Surveying, printed 1774, i" *to. 



4a2 



