
Parr IL Sor. ii. §2] | SILURIAN. 685 
(Orthoceras annulatum, O. tenuicinctum, &c.). In some of the shales 
crustacean fragments are numerous. They include large pieces of the 
carapace of Dictyocaris, with remains of Veratiocaris and Pterygotus. The 
pebbly grits contain Petraia and crinoid stems. In the south of Kirk- 
eudbright certain limestones and conglomerates intercalated among these 
shales have yielded a more varied fauna, having on the whole a decidedly 
Wenlock character, and including Favosites, Catenitpora, Beyrichia tuber- 
culata, Phacops caudatus, Meristella, Leptzna sericea, Atrypa reticularis, 
Strophomena imbrex, Murchisona, Orthoceras tenuicinctum, &c. 
It is impossible in the south of Scotland to separate the Upper 
Silurian rocks into Wenlock and Ludlow groups. On the whole these 
rocks seem to be representative mainly of the older half of the Upper 
Silurian divisions. They are covered unconformably by Lower Old 
_ Red Sandstone and later formations. In the counties of Edinburgh and 
Lanark, however, the base of the Lower Old Red Sandstone is found 
to graduate downward into a thick series of brown, olive, and grey 
shales, sandstones, and grits, containing undoubted Ludlow fossils. It 
is deserving of remark also that the peculiar lithological type so 
characteristic of the strata in the original Silurian area reappears in the 
centre of Scotland, many of the concretionary brown shales and olive- 
coloured mudstones being undistinguishable from those in the typical 
sections at Ludlow. Some of these beds are crowded with fossils, 
among the most typical of which are Leptzna transversalis, Orthonota 
amygdalina, Platyschisma helicites, Beyrichia Kloedent, Orthoceras Maclareni, 
with many crustaceans of the genera Ceratiocaris, Dictyocaris, Eurypterus, 
Pterygotus, Slimoma, and Stylonurus. In the Pentland Hills these strata 
are estimated to attain a thickness of 3500 to 4000 feet, but their base is 
nowhere reached ; in Lanarkshire they are at least as thick. Their lower 
portions may represent some of the higher parts of the Wenlock group. 
Ireland furnishes some interesting evidence regarding the geo- 
graphical changes in the west of Europe between the close of the Lower 
Silurian and the beginning of the Upper Silurian period. It has already 
been pointed out that the metamorphosed Lower Silurian rocks of the 
Scottish Highlands are prolonged into the north of Ireland, whence they 
range south-westwards to Galway Bay. In the picturesque tract 
between Lough Mask and the mouth of Killary harbour these metamor- 
phosed rocks are unconformably overlaid by masses of sandstones, con- 
glomerates, and shales more than 7000 feet thick, and containing 
Llandovery and Wenlock fossils with a mixture of Caradoc forms. In 
the midst of the greatly metamorphosed Lower Silurian platform, 
portions are to be found still little altered and full of fossils. The 
overlying Upper Silurian strata have not been metamorphosed, but 
contain pebbles of the altered rocks on the upturned edges of which 
they lie. It is evident therefore, as Mr. Hull has remarked, that the 
metamorphism must have occurred between the formation of the Lower 
and that of the Upper Silurian rocks of the region.’ In connection with 
this question it should be remarked that abundant volcanic activity 
accompanied the deposit of these Upper Silurian rocks in the west of 
Ireland, successive sheets of lava (eurite) and beds of tuff forming con- 
spicuous bands among the stratified rocks, and reaching a collective 
1 Physical Geology of Ireland, p. 22; Kinahan’s Geology of Ireland, chap. iii. ; 
Geological Survey of Ireland, Explanation of Sheets (76, 77, 83, and 84). — 
