The Scottish Naturalist. 3 1 9 



to something like 330 square miles. The geology is simple, 

 the whole group belonging to the Old Red Sandstone. 



With the exception of Hoy, all the islands are occupied by 

 the lower or flaggy strata of that formation, and are correlated 

 by geologists with the higher subdivisions of Caithness. Hoy 

 fortunately differs from that " land of flatness, flags, and fossil 

 fishes," being principally composed of "coarse siliceous, red and 

 yellow sandstones, and marls," which belong to the upper Old 

 Red Sandstone. An arenaceous series, in appearance much re- 

 sembling the red and yellow rocks of Hoy, but placed in the 

 lower division of the formation, occupies nearly the whole of 

 Eday, and occurs in Sanday, Stronsay, and Shapinshay. It 

 also composes a large portion of most of the South Isles, and 

 crosses the Mainland in a narrow belt from Inganess to the 

 church of Orphir. Granite and gneiss crop up N.W. of Strom- 

 ness, and in Graemsay ; and several dykes of basalt occur 

 throughout the islands, while two volcanic necks are found in 

 Hoy, and an ancient lava flow is seen near the foot of Shapin- 

 shay. 



As may be expected from the geology, there is a sameness in 

 the physical features : most of the islands showing a low undu- 

 lating outline, terminating seawards in a bold cliff, or sloping 

 down to a sandy bay. In Rousay and Westray remarkable 

 terraced hills of rusty-coloured flags occur, due to the denuda- 

 tion of the softer parts in pre-glacial times. In Hoy the hills 

 rise to over 1500 feet, gashed by narrow valleys, with numerous 

 inland cliffs. " Here no man dies, for all their necks are broke." 

 Here, too, are " many comical plants," as a native once informed 

 Dr Boswell ; and, in reality, here is the only glimpse of an alpine 

 flora. 



From Hoy in the S.W., the islands gradually decline in alti- 

 tude towards the N.E. The Ward Hill of Orphir attains 

 876 feet, while Rousay rises to 732 feet, Westray to 549 feet, 

 and Eday to 310 feet — the remainder of the islands being 

 decidedly flat. 



The soil for the most part consists either of peat or of a clay- 

 ish loam, formed by the disintegration of the flaggy rocks. The 

 island of Sanday, as its name implies, is largely covered with 

 blown sand. Several " links," usually of small extent, are met 

 with throughout the islands. 



The average yearly rainfall is 37.22 inches, — May, with 1.73 

 inches, being the driest, and October, with 4.85 inches, the wettest 



