140 The Insh Naturalist. 



We next come to a ver}^ distinct deposit which has been 

 distinguished by the name of estuarine cla}^ and which is the 

 bed to which I wish particularly to draw attention. At Belfast 

 and elsewhere, where the series is well developed, it consists of 

 two zones, differing considerablj^ both in lithological features 

 and in the character of its fauna. The lower zone consists of 

 brownish-blue sand}^ marine cla}', crammed with littoral shells 

 of a limited number of species, of which Scrobicidaiia pipei^ata^ 

 Tapes deciissattis, Cardmm edule, and Mytilus edulis predominate ; 

 the former two of these are now very rare in the north-east of 

 Ireland. This Scrobicularia zone is ver}^ persistent through- 

 out the district under consideration ; it also occurs in Dublin 

 ba}^, and has been recognised in England and Scotland. 



The upper, or Thracia zone is of a different nature. The 

 depression that had caused the submergence of the peaty 

 land-surface and the deposition of littoral sand and mud on 

 the top of it, then assumed a more rapid aspect; in passing 

 from the lower to the upper zone of the estuarine clay we 

 pass from a littoral fauna into one which would be found in a 

 depth of five to ten fathoms of water. Cardium echinatiim^ 

 Scrobic7cla7'ia alba, Liicinopsis tcndata, Montacuta bidentata, 

 Tlwacia convexa, Turritella terebra, Aporrhais pes-peleca7ti, 

 Scalaria tiirtonce, now become characteristic species, and a 

 varied and exuberant fauna, pertaining to the laminarian and 

 coralline zones, replaces the limited littoral fauna of the Scro- 

 bicularia clay. The beds of sand and clay which overlie the 

 Thracia zone in the t3^pical section taken, are again of a 

 littoral character, and attest the re-elevation of the surface 

 to its present position: they are the very latest geological 

 deposit in our CvStuaries, and are, in fact, still in course of 

 formation. 



The estuarine beds, then, show a well-marked series of 

 oscillations of level. The peat represents a period far back 

 in Post-tertiar}^ times (but long after the grand series of 

 depressions and elevations that characterised the ** Great Ice 

 age"), during which the land .stood higher than at present, 

 possibly only a few feet higher in some places, but certainly 

 20 to 40 feet at other spots. An era of gradual depression 

 ensued, accompanied by a deposition of littoral mud on the 

 former land surface. This was succeeded hy a further depres- 

 sion, which submerged the former surface to a depth of 50 to 

 80 feet. The final elevation which succeeded, amounted to 30 

 or 40 feet, and brought about the existing state of things. 

 This series of oscillations is the latest of which we have any 

 geological record, and occurred, in part at least, within the 

 human period. 



The raised beaches and raised sea-beds of the north-east are 

 contemporaneous more or less with the Thracia beds. The 

 same amount of depression which would account for the 

 presence of a 5 to 10 fathom fauna in the clays, at the levels 



