6 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



and, as the distance is nearly four miles, their entire thickness must 

 be at least fully 1500 feet. 



II. Geological Structure of the Strath Valley. 



Coast-sections. — The previous enumeration of the liassic strata of 

 Strath is in reality a description of the beds seen in section along: the 

 north-eastern shore (PL I. fig. 2). They follow each other regularly, 

 from the sandstones and coral-bed of Lussay up to the Scalpa shales, 

 with no material disturbance from faults or igneous rocks. The struc- 

 ture of the south-western coast-line is somewhat more complex. 



At the head of Loch Slapin (fig. 4), among the roots of Beinn Chro 

 and Beinn Dhearg Mor, there occurs a set of limestones and shales 

 occupying the same horizon as those of Breakish. In the bed of the 

 stream that descends between these two mountains a few yards of 

 altered limestone abut against the syenite of the hills ; this is followed 

 by a series of indurated shales with occasional fragments of Belem- 

 nites ; and further down the water-course, a hard bluish-grey and 

 streaked metamorphic limestone becomes the prevailing rock. The 

 dip of these beds is S. by W. at angles of from 25° to 60°. They are 

 capped on the shore at Torrin by a small patch of shale, seen only 

 at low-water, which may possibly represent the under portion of the 

 Pabba series. Following the eastern shore of Loch Slapin, we find 

 the dip turning round to the north-west, and the same series of altered 

 limestones again presenting itself. The bedded structure of the rock 

 is nearly obliterated ; but, where the angle of dip can be observed, it 

 increases southwards beyond the Torrin promontory, until the shore 

 is covered by a great protrusion of syenite, which stretches eastwards 

 for three miles, forming the long ridge of Beinn an Dubhaich. It 

 extends along the shore for about half a mile, and, at the promon- 

 tory south of Camus Smalaig, is succeeded by the limestone as 

 before. South of this junction the beds dip S.W. at 15°-25°, where 

 the bedded structure begins to reappear. They gradually lose their 

 metamorphic aspect, till at the mouth of the stream which descends 

 from Glen Kilbride, they pass into a coarse shelly limestone abound- 

 ing in GryphcEce. The grits and limestones, which for a short space 

 succeed, are the equivalents of similar beds between Broadford and 

 Corry ; and, as in that locality, so also here, they are surmounted by 

 a thick set of dark micaceous shales, identical in character and fossils 

 with those of Pabba. They stretch into the interior for upwards of 

 a mile, whence they deflect to the south, and reach the shore of 

 Loch Eishort at Borereg, — thus occupying the tongue of land which 

 separates the two lochs. At Suishnish Point they are covered by a 

 patch of yellow calcareous sandstone, which, at one part, displays a 

 rude grotto with pendent stalactites, like not a few other caves formed 

 by the decomposition of trap-dykes along " Slapin' s caverned shore." 

 This lip of sandstone is succeeded by the same series of shales 

 dipping N.W. at 3°-5°. Half a mile from Suishnish they are inter- 

 rupted by a large mass of augitic greenstone, which has broken 

 through and overflowed them atop. The headland of Cam Nathrach 

 is formed by this protrusion of greenstone surmounted by another of 



