Chap. VIII.] 
SILURIAN ROCKS OF AYRSHIRE. 
157 
tolites. Here, also, we have Orthoceras angulatum, Wahl., an Upper 
Silurian form. On the other hand, there is a large species not to be dis- 
tinguished from 0. vaginatum, Schloth., so common in the Lower Silurian 
of Scandinavia and Russia, and in addition 0. bilineatum, Hall, a Lower 
Silurian fossil of America. The genus Cyrtoceras is also found here ; and 
with these Cephalopods occurs a thin, finely striated Discina, the D. crassa 
of Hall, which is found also both in the Caradoc and the Llandeilo beds of 
Wales. There are, moreover, double and single Graptolites, both belonging 
to Lower Silurian species. So that in Scotland, as in England and Wales, 
and particularly as we ascend in the series, we meet with rocks in which 
the upper and lower types are mixed together. During the last three 
years the Geological Survey has been at work in Ayrshire ; and Mr. Geikie, 
who has mapped the Girvan district here referred to, has furnished me 
with the annexed section (p. 156). 
Another section, also prepared by Mr. Geikie, shows the detailed relations 
of the rocks in the tract north of Girvan and on both banks of the river. 
Section across the Vale of Girvan at Dailly. 
N.W. Dalquhar- S.E. 
ran Coal- Girvan 
Quarrel Hill. field. Water. • Hadyard Hill. 
a. Carboniferous Limestone with coal-seams, b. Calciferous Sandstone series of the 
Carboniferous Formation, c. Reddish and greenish (Middle ?) Old Red Sandstones and 
Conglomerates, d. Interstratified felspathic trap-rocks, e. Sandstones, shales, and 
conglomerates (Llandovery), g. Caradoc shales, mudstones, conglomerates, and lime- 
stone, h. Shales and greywacke : Caradoc or Llandeilo. /, /. Faults. 
Although this is no place for lithological details, a few words must be 
said of a very striking conglomerate which lies low in the order of the 
strata. Among its rounded and waterworn pebbles, varying in size from 
musket-bullets to blocks of two and three feet in diameter, Professor JSTicol 
and myself distinguished upwards of twenty varieties of rock. They con- 
sist of small specimens of earthy greywacke, and larger blocks of hard 
siliceous greywacke, lydian-stone, hornstone, felspar-porphyries of various 
colours, greenstone, syenite, and granite. 
Such conglomerates, which may be indicative of the powerful and long- 
continued action of waves on a coast, are, however, only to be viewed as 
local phenomena, and may therefore be looked for in various parts of this 
ancient series in Scotland, just as they have been shown to occur at 
various levels in the Silurian rocks of England and Wales. They occa- 
sionally appear, indeed, in the coarse grits of the Longmynd or bottom 
rocks; and coarse conglomerates occur in the same unfossiliferous sub- 
jacent greywacke (Cambrian) of Carnarvonshire. 
The shelly Silurian beds of Ayrshire being covered towards the north by 
