18G5.] BRTCE — ARRAX DPaFT-BEDS. 209 



tacts and tlie complete distinction between the contiguous beds. 

 The lowest bed, or true Boulder-clay, has its usual character, and 

 does not yield a single shelly fragment ; directly the overlying clay 

 is reached, the shells become abundant. The shell-bed has the same 

 structure and contents as in the other sections ; over it are the Upper 

 Drifts, strongly contrasting with the Boulder-clay, but less com- 

 pletely shown here owing to the nature of the ground. 



This section is highly illustrative of a case which is very apt to 

 mislead an observer, especially when it occurs among basin-shaped 

 or level deposits. It often happens that the beds thin out at the 

 outcrop as rocky strata do, either from an inequality in the original 

 deposition or from their successive erosion. The beds are thus 

 brought so close together that the distinction passes unobserved, and 

 shells from an upper bed get on to the surface of a lower, or perhaps 

 a little way into its substance ; on a gently sloping bank by the sea, 

 or by a river-side, the shell-bed might be swept off and its fossils get 

 into the Boulder-clay forming the floor, and thus appearances the 

 most misleading be produced. 



The following list contains the species found by the Rev. R. B. 

 Watson in the Arctic shell-beds of Arran : — 



Balanus crenatus. Cyprina Islandica. 



Purpiu'a Norvegica. Leda pygmaja. 



Astarte elliptica. pernula. 



arctica. Pecten Islandicus. 



compressa. opercularis. 



striata. Litorina litorea, 



Cryptodon Sarsii. Tumtella communis 



Modiola modiolus. Natica (?). 

 Tellina Baltica. 



Of Foraminifera and Entomostraca the following species, named 

 by Mr. Rupert Jones, were found by Mr. Crosskey and myself; there 

 are, however, several others which Mr, Jones has not yet had time 

 to name : — Rotalia Beccari, PoJystomella striato -punctata, Gyihere (a 

 new species closely allied to G. LamarcMana, an Atlantic living form 

 undescribed). Besides these, we found also Cardium fasciatum, Rissoa 

 globulus, Serjpula spirillum, and fragments of Echinus (species not 

 determinable). 



Comparison with the Clyde Beds. — When the beds we have jiist 

 been describing are compared with those which form the Clydesdale 

 series, some remarkable differences appear. The most remarkable 

 feature is the constant presence of a laminated clay in the basin 

 of the Clyde, dividing the Boulder-clay from the Arctic shell-bed. 

 It is a fine fissile clay, easily opening up into thin laminae like 

 the leaves of a book ; it is unfossiliferous and entirely without 

 stones — even the trace of a pebble. It is thus in most striking 

 contrast with the Boulder-clay on which it rests. It has been 

 already shown that the shell- bed in Arran rests on the Boulder- 

 clay directly, the laminated clay being absent. The state of the 

 shells presents another contrast. Those of the Arctic bed in Clydes- 

 dale are generally entire, even delicate shells being well preserved ; 

 while in Arran the shells are generally fragmentary, at least there 



q2 



