GEOLOGY OF LESMAHAGO. 51 
(traversed on the dip) of about four miles; the uppermost bed containing Lengula cornea, 
Platyschisma helicites, Beyrichia, &c. 
“Dictyocaris Slimoni also occurs in these beds, but is never obtained entire. Fragments 
of Trilobites (?),a few small shells, and some supposed vegetable remains, complete the list 
of fossils. 
“Tn many of the beds seen in this section no fossils of any kind are found. 
“These grey shaly and slaty beds dip beneath and are conformable with a series of red 
and grey shales of a more sandy nature than the foregomg (but unfossiliferous), 
extending for a mile and half; I have called them “ Red Silurian,” but Sir Roderick 
Murchison considers they should form the base of the Old Red Sandstone series. In one 
bed only fragments of a Stylonurus have been found, and one small specimen, thought to 
be part of a Simonia acuminata, but the evidence is too fragmentary to rely on. All the 
strata are very much disturbed, owing to intrusions of felspar- and hornblende-porphyry, 
causing many of the beds to be repeated and faulted.” 
It is, however, to these very disturbances that we are, no doubt, indebted for the 
exposure of the Pferygotus-bearing shales of Logan Water. 
The subjoined section is copied from one which accompanies Mr. Geikie’s paper 
published in the ‘Quarterly Journal of the Geological Society,’ 1860, vol. xvi, pl. xviii, 
Section 1, and exhibits the Pterygotus-bearing shales as they appear in the anticlinal of 
the bed of the Logan Water, and the succeeding Lower Old Red Sandstone and Carboni- 
ferous deposits. 
Nethan River 
S.W. Logan Water. Logan Water. Water. Black Hill. Clyde. N.E. 
1 
1 
1 
' 
1 1 1 
' 
J 
; } : ie ie 
Upper Silurian. Lower Old Red Sandstone. 
Fic. 7.—Section of the Upper Silurian and other strata near Lesmahago. 
a, a. Purplish-grey Sandstones. 6, b. Red shales, Sandstone bands, and Conglomerate. c, e. Olive shales. 
d, d. Hard flaggy shales (Pterygotus). e. Hard shales and stone bands. (jf, f,f. Felstone. 
C, C, C. Carboniferous strata (unconformable to Old Red). 
Mr. Arcuipatp Grrxin, F.R.S., the Director of the Geological Survey of Scotland, 
has most obligingly furnished me with the following descriptive section, in descending 
order, of the deposits of the Lesmahago district, taken from the joint observations of his 
colleague Mr. B. N. Peach and himself. 
