236 R. N. WORTH ON CERTAIN ALLUVIAL DEPOSITS 



27. On certain Alluvial Deposits associated with the Plymouth 

 Limestone. By B,. N. Worth, Esq., F.G.S. (Read April 5, 

 1876.) 



In the ingenious paper on the Drift of Devon and Cornwall, 

 read by Mr. Belt before this Society in November last, that gentle- 

 man speaks of the lowland gravels of Devon and Cornwall as show- 

 ing " signs of sudden and tumultuous deposition." Whether he is 

 right or wrong in this assumption can best be settled by a reference 

 to detail in various localities ; and it is with the view of aiding in 

 this work that I would direct attention to certain alluvial deposits 

 associated with the Plymouth limestone, which seem, so far as that 

 locality is concerned, to negative Mr. Belt's hypothesis. 



The limestone of Plymouth, and that of Yealmpton (which com- 

 mences about two miles distant from the eastward termination of the 

 first), abound in fissures and caverns. Some of the latter have become 

 celebrated by their ossiferous contents. Some of the former contain 

 the alluvial deposits to which I refer, and which, denuded off else- 

 where, are therein preserved. 



The best examples of these deposits have occurred in connexion 

 viith Plymouth Hoe, where the so-called drift-gravel fills sundry 

 fissures, and has been found to entomb remains corresponding very 

 closely to the fauna of the Oreston caverns, which are less than a mile 

 distant, on the other side of the Cattewater. 



The chief deposit occurs about midway on the Hoe, and fills a 

 huge " pocket " in the limestone. Beneath the turf there is a bed 

 of clayey soil containing pebbles, which range from a very small size 

 up to boulders a dozen pounds or more in weight. They are chiefly 

 quartzose; some composed of quartz and schorl, others granitoid, 

 and a few of slate. But for the absence of granite, this part of the 

 deposit in no respect differs from the ordinary alluvium of our Dart- 

 moor rivers. 



Beneath this pebble-bed are patches of white and red clay, con- 

 taining few pebbles ; and beneath this, again, a large quantity of 

 siliceous sand, the depth of which has not yet been ascertained, as 

 the excavations have not reached the bottom of the fissure. The 

 sand is chiefly of a cream- colour. 



Similar deposits to this series are by no means of unfrequent oc- 

 currence in association with the limestone, though sometimes the 

 sand, sometimes the clay, and sometimes the pebbles appear to pre- 

 dominate. They are found not only on the Hoe, but at Cattedown 

 (where, in Deadman's Bay, is a " pocket " at least 40 feet in 

 depth, containing white clay similar to that described), at Billa- 

 combe (near the termination eastward of the Plymouth limestone), 

 and at Yealm Bridge (where a fissure contains pebbles identical 

 in character with those in the bed of the Yealm, 50 feet below). 



