82 Proceedings of the Boyal Physical Society. 



succession on the eastern side of the Mainland as well as the 

 relations of the associated contemporaneous and intrusive 

 igneous rocks in the western districts. 



While pursuing this object we were fortunate enough in 

 discovering in the Walls district a rich series of plant remains 

 in rocks which have been hitherto considered as forming 

 part of the metamorphic series. Mr C. W. Peach has kindly 

 named the plant remains for us, and from his description it 

 is evident that they are identical with the plants found in the 

 Old Eed Sandstone formation of Caithness and Orkney. 

 The rocks in which they are embedded must therefore be 

 relegated to that period, though they seem to have under- 

 gone a considerable amount of metamorphism. 



In this paper we propose to give a brief sketch of the 

 different areas occupied by these rocks in Shetland, indicat- 

 ing as far as possible the succession of events and the rela- 

 tions of the contemporaneous and intrusive igneous rocks. 

 We shall endeavour to show that during the early phases of 

 that period, the Mainland, which is the largest of the Shet- 

 land group, must have formed an island somewhat smaller in 

 size than now, round whose coast-line the basement breccias 

 accumulated ; but eventually as the land slowly sank beneath 

 the sea-level, the higher deposits overlapped on to the gneissose 

 rocks, and ultimately buried them. The long process of 

 denudation to which the Shetland archipelago has been 

 subjected has removed in a gTcat measure the greater portion 

 of these deposits ; those which now remain being protected 

 in part by the liard gneissose rocks against which they have 

 been brought by dislocations of the strata. 



The order of succession on the east side of the Mainland is 

 as follows : 



e. Flaggy group of Bressay and Noss. 

 d. Lerwick sandstones. 

 c. Eovey Head conglomerates. 

 h. Brenista flags. 



a. Basement breccia, resting unconformably on the under- 

 lying schists. 



Owing to a series of faults which form the boundary line 



