600 Proceedings of the Royal Society 
the flagstone series, the protruding ridge of granite and gneiss 
which rises at Stromness and Gremsa being merely an indication 
of the irregular surface on which the deposits of Lake Orcadie 
were accumulated, and of the slow progressive subsidence of the 
area. The fossils, for which these islands have long been famous, 
include most of those of the upper groups of Caithness, with the 
addition of others which have been regarded as distinct. In the 
determination of these fossils, much skill is required to discriminate 
between the accidental differences of aspect resulting from the con- 
dition of fossilisation. The Orkney fishes, for instance, are pre- 
served as black jet-like impressions which, often very perfect when 
first removed from the quarry, are apt to scale off, leaving in each 
case only an amorphous layer which, though it retains the contour 
of the fish, shows little or no trace of structure. On the shores of 
the Moray Firth, on the other hand, the organisms have been 
inclosed within calcareous nodules ; their colours are sometimes 
brilliant, and their scales, plates, fins, and bones are often admirably 
preserved and remain unchanged in the Museum. Want of 
experience in these different modes of preservation may have led to 
a reduplication of species, especially in the case of the Orkney 
and Moray Firth fishes. Among the most interesting Orkney fossils 
is a portion of a Pterygotus (recognised by Dr H. Woodward), now 
in the British Museum. The occurrence there of this characteristi- 
cally Upper Silurian and Lower Old Bed Sandstone genus supports 
the view contended for in this paper as to the true horizon of the 
Orkney and Caithness flagstones. 
The Shetland Islands contain a portion of the shore-line of Lake 
Orcadie, with its conglomerates and sandstones and the flagstones 
and shales of deeper water. Among these strata the Caithness 
Estheria occurs, with abundant stems and roots of large calamite-like 
plants with well-marked flutings, but without observable joints. 
Some ichthyolites of the Caithness type are said to have been found 
in Bressay. The general lithological characters are quite those of 
the sandy parts of the Orkney and Caithness groups. On the west 
side of the mainland of Shetland interesting evidence occurs to show 
the existence of volcanic action contemporaneous with the accumu- 
lation of the Old Bed Sandstone. Beds of amygdaloidal lavas and 
bands of tuff occur among the sandstones, the whole being pierced 
by masses of pink felsite. 
