POST-TERTIARY POSSILIFEROUS DEPOSITS. 49 



of marine and arctic character, without the least admixture of those decided brackish- 

 water forms so common in the present neighbouring lagoons of the Clyde. 



4. At the south end of the hollow, lying between the shell-clay and the upper mould, 

 a bed of brown clay is disclosed two feet in thickness which has never been disturbed 

 since its original deposition. 



As all these circumstances are incompatible with the supposition that this deposit has 

 been interfered with by human agency, they are equally so with the theory that it has 

 been drifted from any near or distant locality by the action of ice. The fossils bear no 

 trace, moreover, of having been in any way rolled or crushed, as they would infallibly have 

 been from such a cause. 



Traces of a thin irregular bed of small shells have been seen near the surface along 

 the south-east side of the dock ; and about one hundred yards from this, in an easterly 

 direction, another shelly deposit, belonging to the same series, has been exposed on the 

 side of a sandstone quarry, where the shells and other organisms, though fewer in 

 number and variety of species and different in their proportions, are similar in character 

 to those we have described. 



Another shell-bed, also of Arctic type, has been brought to light in making a channel 

 way to the west end of the dock, rather below half-tide. That these beds at one time 

 were all connected is highly probable ; but there appears no evidence that any of them 

 were ever covered by the Boulder Clay. 



At first sight the deposit in the hollow, indeed, seems to dip under the Boulder Clay, 

 but on further examination it is clearly seen to thin out to the surface. 



The whole Frith of Clyde on both sides is patched with beds of laminated 

 fossiliferous clay, which, doubtless, at one time covered the bottom from side to side, 

 and reached the various heights on which we find their remains above the level of the 

 sea. The whole deposit has evidently sufiered much since it was first laid down, from 

 currents, changes of level, and other causes. 



The clays are generally found cut away between low tide and high-water mark, 

 where the abrading power of the water is greatest. Examples may be seen at Langbank, 

 Helensburgh, Roseneath, Fairhe, Cumbrae, &c. In these localities the truncated edges 

 of the fossiliferous clays are exposed to view here and there in the more sheltered hollows 

 along the tidal belt. 



The trough at Cartsdyke has, doubtless, been separated from the deposit existing 

 close by, near low-water-mark, by the agencies determining the distribution of the beds 

 through the whole Erith. 



The Cartsdyke deposit is remarkable, not only for its puzzling appearance, but 

 for the great abundance and diversity of organisms found crowded within its narrow 

 limits. 



The dry fossiliferous clay consists of 76 per cent, fine mud; 19 per cent, fine and 

 coarse sand ; 5 per cent, gravel and shell debris. 



7 



