32 POST-TERTIARY ENTOMOSTRACA. 



recur, although the bores show that in some places sand alternates with gravel to a 

 depth of eighty or ninety feet. 



The layers exposed in the railway-cutting do not maintain a horizontal line or follow 

 a regular dip, but present false beddings with violent contortions. 



The overlying shell-bearing clays conform, however, to the sand banks over which they 

 are deposited, with depressions less or more abrupt according to their character. The 

 shell-bearing clays of this section are therefore evidently of later date than the estuarine 

 sands upon which they rest, as well as later than the Boulder Clay. 



The clay is for the most part of a sandy character ; it is finest towards the east, where 

 it is deepest and overlapped with muddy sand, with which it is less and less mixed as it 

 descends. 



Along the north of the cutting, where the clay is seen overlying the sand, many of the 

 layers of sand are more or less mixed with mud, showing that the deposit of mud has 

 gone on mixing with the sand until changed circumstances permitted the mud to 

 accumulate. 



The divisions between the layers of clay are, in most instances, formed by a very 

 fine sprinkling of fine sand, and groups of these layers are often again divided by thicker 

 and coarser beds of sand. 



These groups of thin layers are sometimes nearly of the same depth, but at other 

 times are more irregular, and the bands of sand vary in thickness. 



These very thin layers appear to have been formed by frequent and gentle undula- 

 tions of the water. Winds and tides would carry over the mud only the finest particles 

 of sand, while the thicker bands would be produced by stronger winds and storms. The 

 lapse of time between the recurrence of these disturbing causes may be reckoned by the 

 distance of the bands from each other, and their strength by the depth of the bands. 



Prora the sharp lines between the bands we may infer that the causes were transient ; 

 but in the case of such deposits as those at Stobcross, where the sands and clays have 

 been washed into each other, the conditions must have been more continuous and the 

 water comparatively shallow. 



The fauna (which has been chiefly obtained from the east bank of clay), although not 

 abundant, furnishes ample proof of the purely marine character of the deposit. Amono- 

 the shells obtained were — 



Tellina calcarea, 

 Cyprina Islandica, 

 Mya truncata, &c. 



In the muddy sand exposed at a lower level in the excavation for the Stobcross" Docks, 

 close by the Clyde, brackish water Ostracoda are found, marking the change from a sea to 

 an estuary. 



