94 ■ PSfeOCEBDINGS OF THE GEOLOGICAL SOCIETY* 



stone north of the Grampian chain/ There appear to have been 

 at least three distinct and widely-separated groups of volcanoes in 

 the great lake or inland sea in which the thick pile of the Caithness 

 Flags was deposited, and which for the sake of brevity of reference 

 I have named 'Lake Orcadie/ Not far from the southern shore, 

 two or mo-re small vents made their appearance. Their exact posi- 

 tion has not been ascertained, but the basic lavas which proceeded 

 from them have been traced on the maps of the Geological Survey. 

 These diabases are well exposed in the long narrow valley of 

 Strathbogie, where they have been mapped for the Geological Survey 

 by Mr. L. Hinxman ; ^ while 25 miles to the north, and doubtless 

 belonging to another vent, a smaller patch has been traced by 

 Mr. J. Grant Wilson near Buckie.^ 



At a distance of some 90 miles northward from these Moray Pirth 

 vents a second volcanic district has been detected among the Orkney 

 Islands by Messrs. Peach and Horne.'^ The lavas which were there 

 ejected occur at the south-eastern corner of the island of Shapinshay, 

 where they are regularly bedded with the flagstones. They consist 

 of dark green amygdaloidal olivine-diabases. Their thickness can-, 

 not be ascertained, as their base is not seen, but they have been 

 cut by the sea into trenches which show them to exceed thirty 

 feet in depth. The position of the vent from which they came has 

 not been ascertained. Neither here nor in the Moray Firth area 

 do any sills accompany the interbedded sheets, and in both cases 

 the volcanic action would seem to have been of a feeble and short- 

 lived character. 



Much more important were the volcanoes that broke out nearly 

 100 miles still farther north, where the Mainland of Shetland now 

 lies. I shall never forget the pleasure with which I first recognized 

 the traces of these eruptions, and found near the most northerly 

 limits of the British Isles proofs of volcanic activity in the Lower Old 

 Eed Sandstone. Since my observations were published,^ Mr. Peachy 

 who accompanied me in Shetland, has returned to the district andy 

 in concert with his colleague Mr. Home, has extended our know- 

 ledge of the subject.® The chief vent or vents lay towards the west 



1 Trans. Eoy. Soe. Edin. vol. xxviii. (1878) p. 345. 

 ^ Sheet 76 of the Geological Survey Map of Scotland. 

 ^ Sheet 95 of the same map. 

 * Proe. Eoy. Phys. Soc. Edin. vol. v. (1879) p. 80. 



5 Tj,^ng. Koy. Soc. Edin. vol. xxviii. (1878) p. 418. 



6 Ihid. vol. xxxii. (1884) p. 359. ... 



