﻿GEOLOGY OF LESMAHAGO. 



51 



(traversed on the dip) of about four miles ; the uppermost bed containing Lingula cornea, 

 Platysclrisma helicites, BeyricJiia, &c. 



"Dicfyocaris 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. 



" In 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 foregoing (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 Stylonicrus have been found, and one small specimen, thought to 

 be part of a ISlimonia 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 Pteryyotiis-hearmg 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, pi. xviii, 

 Section 1, and exhibits the Pteiyyotus-hearmg 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. 



s.w. 



Losan Water. 



Lomii Water. 



Nethan 

 Water. 



Black Hill. 



River 

 Clvde. 



N.E. 



Upper Silurian. 



c. b. a. f. 



Lower Old Red Sandstone. 



/• 



Fig. 7. — Section of the Upper Silurian and other strata near Lesmahago. 

 a, a. Purplish-grey Sandstones. b, b. Red shales, Sandstone bands, and Conglomerate. c, c. Olive shales. 



d, d. Hard flaggy shales (Pterygotus). e. Hard shales and stone bands. /,/,/• Felstone. 

 C, C, C. Carboniferous strata (unconformable to Old Red). 



Mr. Archibald Geikie, 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. 



