GLACTATION OF THE OEKNEY ISLANDS. 661 



flints, fossil wood, pink granite, porphyritic felstone, &c., all of 

 which are foreign to the islands. 



According to the opinion of Mr. Carrathers, F.K.S., the blocks of 

 dark-grejr limestone with plant-remains in all probability belong 

 to the Calciferous Sandstone series. He has identified a well-marked 

 specimen of Lepidostrohus in one of the blocks, though it is not 

 distinct enough to be named specifically. Lithologically the boulders 

 resemble some of the thin limestone bands in the Cement-stone series 

 of Central Scotland ; and the nearest locality to Orkney where thes^ 

 rocks occur in situ is in the county of Fife. With reference to the 

 Secondary rocks, Professor Judd, F.E.S., states that, besides the 

 chalk and chalk- flints, he detected amongst our collection some 

 specimens which resemble some of the Secondary rocks of Scotland. 

 Two specimens of the calcareous breccia from the Boulder- clay in 

 Odin Bay "very closely resemble parts of the Upper Oolites of 

 Sutherland," and two other blocks are probably from the same 

 locality. Moreover, he adds that the specimens of oolitic limestone 

 very possibly come from some part of the Secondary series in 

 Scotland. 



In addition to these, we observed, in the Odin-Bay section, 

 large blocks of a remarkable rock which seems to be petrified wood. 

 It has a curious fibrous structure and is very calcareous ; indeed 

 under the microscope it appears to be mainly made up of crystals 

 of calcite, though occasionally there are portions where the struc- 

 ture is still retained. Blocks of the same rock, however, occur in 

 the Caithness Boulder-clay, which show traces of organic structure 

 under the microscope. On dissolving a small piece of the rock a 

 large residue of coaly matter was obtained, which ignited with a 

 strong flame. It would appear that this rock is largely burnt for 

 lime in Sutherlandshire, where it is washed out of the Oolitic 

 shales. 



In all probability most of the blocks of granite, felstone, 

 gneiss, quartzite, and schist which occur throughout Orkney, save 

 those in the Stromness district, have been derived from the north- 

 east of Scotland, though they possess no special characteristic^ 

 which might enable us to identify them with any particular 

 locality. 



Now it ought to be borne in mind that chalk, chalk-flints, and 

 various rocks of Jurassic age are found in the Boulder-clay of 

 Caithness, and also in the same deposit in the low grounds of Banfi- 

 shire and Aberdeenshire, where it possesses the same physical cha- 

 racters as in Orkney, and likewise contains fragments of shells. It 

 seems perfectly reasonable to conclude, therefore, that the Boulder- 

 clay in these widely separated localities must be ascribed to a 

 common cause, or, in other words, to the action of land-ice. Indeed 

 no one who attentively examines the sections in Orkney would 

 ascribe them to the action of icebergs or coast-ice. We have already 

 discussed the objections to the marine origin of the Shetland till, 

 and the very same arguments apply with equal force to the present 

 case. 



