524 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 24, 



and strong and thick, but very compressed in form. A fourth (of 

 which, however, I have got nothing but small fragments) is a bivalve 

 with strong concentric growth-lines, having the interstices ornamented 

 with peculiar radiating striae. It is also crenated along the ventral 

 margin similar to the Venus Casina. Some of the bits have evidently 

 belonged to a good-sized shell, and are thickened along the edge, 

 while others are the remans of individuals of smaller size, or perhaps 

 of a different species. I have found it in many different places. 

 Fragments of Astarte and Pecten are also not uncommon. 



I have stated (p. 516) that, in a cliff on the coast of Slains, and 

 immediately opposite these gravel-ridges, I found in the sandy strata 

 seams of gravel containing similar fragments of shells along with 

 pieces of the same kind of limestone. • It would therefore appear 

 that these mounds are formed out of the same materials, and may, 

 perhaps, be a development of those gravelly layers which are inter- 

 stratified with the fine sand. They may be old submarine banks of 

 gravel, which have subsequently been moulded into the form of ridges 

 by denudation. 



Their base is at about the same elevation as the top of the coast- 

 cliffs, or perhaps a little lower, and they can be traced almost close 

 to the cliffs, while it is also found that a stratum of red clay over- 

 lies both them and the stratified sand at Collieston ; and further, in 

 some places the gravel of these ridges appears to pass into fine sandy 

 layers, similar to the masses along the coast. 



The district in which these shelly gravels occur lies between the 

 estuary of the Ythan and the Burn of Cruden, comprising an area 

 of about 15 square-miles. This tract of ground culminates, with 

 gentle slopes, in an eminence called Highlaw, whose height I found 

 to be 299 feet above the mean level of the sea. The summit con- 

 sists of a mound of gravel, while banks and knolls of coarse water- 

 worn shingle form the most of the adjoining ground. In some places 

 these masses of gravel show many undulating layers and seams of 

 sand, while in others the disposition of the materials is more irre- 

 gular. No broken shells nor fragments of sandstone or limestone 

 occurred to me here. The materials were of gneiss, granite, quartz- 

 ose rock, felspar, porphyry, and other kinds of trap, sometimes not 

 much rounded. It appears then that, while fine sand and mud 

 were accumulating in the deeper recesses of the ancient sea, here, 

 on the shoals and higher portions, under some 40 fathoms less water, 

 the soundings would have shown coarse gravel, sand, and shingle. 



I remarked, however, that a deposit of red clay in many places 

 overlies this gravel, filling up the irregularities and hollows of its 

 surface. It is never of great thickness, and is generally absent on 

 the higher mounds, or forms but a very thin layer over them. 



A similar clay was found in many places overlying the gravel of the 

 Kippet Hills (p. 522) ; and a thick mass of it covers the stratified sand 

 in the cliff at Collieston. Do not all these instances point to a de- 

 pression of the sea-bottom, converting its shoals into sunken banks, 

 and its deeper hollows into yet profounder depths ? 



I did not observe any large boulders beside Highlaw itself, but 



