The Old Red Sandstone of Orkney. 341 



seen, though the sea has cut trenches in the rock at least 

 thirty feet in depth. Where the dip is visible, it is seawards. 

 In the cliff they are covered by the flags, but they crop out 

 inland not far from the coast line. These rocks are dark green 

 in colour, weathering olive green. They may be considered 

 as varieties of diabase, which have undergone considerable 

 alteration. Some of the specimens contain much calcite, 

 which fills drawn-out vesicles, indicating the flow of the 

 molten lava. We have had some of these rocks sliced, and 

 examined under the microscope, which confirms the opinion 

 regarding the extreme alteration which they have under- 

 gone. 



One of the sections is found to be largely constituted of 

 a plagioclase felspar, with a small amount of intervening 

 chlorite, but with much altered olivine. The felspar is much 

 decomposed, and forms the bulk of the rock. The olivine, 

 which is now changed into a pale greenish yellow serpentine, 

 is distributed in large crystalline grains, and is abundant. 

 In places the chlorite is represented by masses of radial and 

 vermicular groups of crystals which appear to have under- 

 gone a change to the same serpentinous mineral as that 

 which replaces the olivine. IMagnetite is irregularly distri- 

 buted as grains, and also frequently, either wholly or partly, 

 envelopes the crystals of olivine. 



Another section shows that the rock consists of closely 

 crystallised plagioclase and much interstitial augite, with a 

 considerable amount of olivine. The augite and the olivine 

 have been converted into serpentine, although a few crystals, 

 as well as portions of crystals, still remain unaltered. The 

 felspar in many places is permeated with the same mineral. 

 Much magnetite is present, together with quartz, some calcite, 

 and serpentinised chlorite.* 



Intrusive Igneous Rocks. — Among the few examples of this 

 class met with are the two necks filled with volcanic as- 

 glomerate already described by Professor Geikie as occurring 

 on Hoy, and which he has shown, in all likelihood, helped to 

 supply the volcanic platform which underlies the main mass 



* In the examination of the microscopic sections we have been kindly aided 

 by Mr T. Davies. 



