20 The Irish Naturalist. February, 



advantage of observing a sunken bog telling of a depression 

 of the land, and at the same place a raised beach testifying 

 to a subsequent exaltation of that precise area. These local 

 terrestrial accumulations do not tell of a deep bog, the 

 result of a continuous and uninterrupted growth of the 

 Sphagnum. They contain great quantities of branches of 

 trees, with leaves and nuts, and the peat is made up to a 

 great extent of marsh plants as distinguished from bog 

 plants. The indications point to a time when low flats 

 fringed the shores of the bay, and when along its sides 

 there were jungles in which sedges, flags and rushes made 

 up the undergrowth, and Willows, Hazels, and Alders the 

 arboreal vegetation. As to the height at which the land 

 then stood we can only give a negative estimate — it must 

 have been at least twenty to twenty-five feet higher than 

 at present. To arrive at this result you must allow eleven 

 feet for the mean rise of the tide, then for the thickness of 

 the peat bed and its dip under the sea allow nine or ten 

 feet more ; its extension downwards below low water we 

 have no means of knowing, but twenty -five feet will, perhaps, 

 be a safe estimate of the elevation of the shore of that 

 period. 



(2). The Scrobicularia period, — This is an exceedingly 

 well marked feature in our estuaiine clays. I am about to 

 rest the proofs of what I now advance mainly on those 

 clays, and it may be well to shortly explain their nature 

 and importance. Estuarine deposits are not peculiar to 

 Tertiary or to recent times, but have been formed in the 

 estuaries of ancient lands during all geological epochs. The 

 Wealden estuary is a well-known example. I will, however, 

 use the term " estuarine clay " to designate those clays 

 formed subsequently to the Boulder-clay in existing 

 estuaries all round our coasts. They are usually tenacious 

 blue clays composed of very fine mud, brought down by 

 rivers, and deposited as silt when the river water loses itself 

 in the sea. They contain very little intermixture of gravel, 

 sand, or pebbles, but in most cases shells are abundant ; 

 and as they are really shells of the period, that have lived 

 and died on the spot, and not waifs or strays cast up by 

 storms, or washed into a certain position by currents, they 



