62 POST-TERTIARY ENTOMOSTRACA. 



On the north side of the burn blue clay is overlain by about a foot of gravelly 

 clay, with boulders of various sizes thickly interspersed, not unlike a shore shingle. In 

 this gravelly clay Mya truncata is exceedingly plentiful in its natural boring position, 

 with both valves united. It also occurs in the underlying blue clay, more than a foot 

 and a half lower down. d 



The burrow of this Mollusc, however, never exceeds half that depth, and consequently 

 the Mya of the upper bed could not have been contemporaneous with the Mya of the 

 lower ; and we have the record of successive generations. 



The fauna of the shell-bed is most abundant ; but the grouping of the shells, like the 

 composition of the deposit, varies at different points. At one spot 3fya truncata abounds, 

 the shells being thickly packed in a sandy matrix, and in many cases having their 

 syphons preserved. At another, this shell is absent, and Saxicava ruyosa stands in its 

 natural position, and Asiarte borealis is found with united valves. The smaller shells, 

 and even the Entomostraca, equally vary in grouping at different points. We have, in 

 fact, all those local varieties of habitat characterising a sea-bottom. There is no sign of 

 any confused sweeping of one part into another. 



The occurrence of the shell-bed in patches may be due either to the irregular 

 character of the surface over which the shell-bearing deposit is spread, or to abrasion 

 from above. 



Of the former, there are good illustrations in many of the brickfields near Paisley 

 in which knolls of Boulder Clay rise up through the laminated clay and at some places 

 reach the surface. Of the latter, there is a good instance on the shore at high-water 

 mark, a little east of Helensburgh, where a patch of shell-bearing clay lies in a rent or 

 gully of Boulder Clay and has been worn down to a level with the beach. 



At one point in the Loch Gilp sections the shell-bed dips under the stream, which is 

 literally paved with Mya truncata in a boring position. 



A few boulders are scattered through the shell-bed; but grooved and polished 

 specimens are not as abundant among them as they are in the clay beneath, although 

 they occasionally occur. The change is almost as great from striated to unstriated 

 imbedded stones, as from the absence of a fauna to its abundant development. 



The boulders in the shell-bed are often covered with SerjmlcB, and evidently, there- 

 fore, must have been stationary during a certain period. They are small in size 

 compared with those in the clay beneath. 



Above the shell-bed in one locality is a patch of fine sand (two to three feet) 

 destitute of organic remains, and this is succeeded by a bed of about the same depth of ■ 

 ferruginous gravel. The whole of these deposits, however, vary in thickness at different 

 points. In some places the sand thins out, in others the gravel disappears and a bed of 

 mere surface-soil covers the shells. _ 



The complete section is : ^ 



