Proceedings of the Roijal Society of Edinburgh. 195 



vibration is obtained, if it be supposed to move in vacuo, and its 

 mass be simply conceived to be augmented by balf that of an equal 

 volume of the fluid, while the moving force with which it is actuated 

 is diminished by the whole weight of the same volume of fluid. 



8. Observations on the Fossil Fishes lately found in Orkney. 

 By Dr Traill. 



The geologist has been for some time acquainted with the occur- 

 rence of Fossil Fishes in Caithness, and they have been more lately 

 found also in Orkney, especially near Skaill, the seat of W. G. Watt, 

 Esq. in Pomona. 



The author describes these fishes as imbedded in a dark coloured 

 flag, which lies beneath three feet of soil and loose stone, and eleven 

 feet of solid beds of similar flag, but destitute of organic remains. 

 The fishes are contained in two strata, measuring together about two 

 feet in thickness. The upper stratum contains only fishes belonging 

 to the Carlilaginei, and seemingly the genus Raia ; the lower con- 

 tains numerous fishes that belong to the orders Thoracici and Abdo- 

 minales, most of them with distinct scales. Almost all of them lie 

 on their bellies or sides, none on their backs, and their attitudes ge- 

 nerally bespeak the energy of their final struggles. The fishes of 

 these two contiguous strata are never intermixed. The strata dip 

 about one foot in seven to the north-west. The author found only 

 a single specimen of a petrified vegetable with the fishes. It was 

 the leaf of a canna or a reed. 



The Orkney Islands have much uniformity in their geological 

 structure. The principal rock is this sort of slate, which is connected 

 with sandstone, and has occasionally interposed thin beds of lime- 

 stone, that seldom contain any organic remains. 



The only primitive rocks in Orkney are in a limited district around 

 Stromness, and in the contiguous small island of Graemsey. There 

 granite, and gneiss approaching to mica-slate, appear in the surface, 

 and have resting on them a coarse sandstone conglomerate. This 

 last is in immediate contact with the slaty rock described above. 

 The highest ridges in Orkney are the mountains of Hoy, which are 

 composed of thick beds of sandstone, in which the author lately dis- 

 covered a vast bed of trap. This sandstone, as well as that which 

 occurs in the other islands, belongs to the old sandstone ; and the 

 slaty rock is probably a newer part of the same formation. 



There are not any distinct traces either of the mountain limestone 

 or of the coal formation in Orkney, unless we are disposed to consi- 

 der this slaty rock as the oldest member of the mountain limestone. 

 But from its connexion with the sandstone, it is safer to reckon it a 

 member of the old red sandstone series. 



Specimens were exhibited to the Society illustrative of the author's 

 statements. 



N ^ 



