544 STEATIGEAPHICAL GEOLOGY 



clay in which Nummulites elegans (var. prestwichiana) is common, 

 and serves to define the Barton from the underlying Bracklesham 

 Beds. Above are grey clays and sands containing Valuta athleta, 

 Cassis ambigua, Rimella bartonensis, and Gominella canaliculata. 



The Middle Barton, or Barton Clay proper, consists mainly of 

 grey and brown clays with bands of septaria, and its prevalent 

 fossils are Valuta luctatrix, V. ambigua, Rostellaria ampla, Murex 

 minax, Glavella longceva, and Grassatella sulcata. At Alum Bay 

 there is a remarkable increase in the thickness of this group, and 

 the upper 70 feet consist of yellowish sandy clays with many small 

 Mollusca, no such beds being seen at Whitecliff. 



The Barton Sands at Barton present the following succession : 



Feet. 

 4. White and yellow sands with Potamides concavus, Olivella 



Branderi, and Melanin hordeacea ...... 20 



3. Dark -grey sandy clay with Olivella Braiideri and other shells . 26 

 2. White sand without fossils ....... 25 



1. Bluish-grey clayey sand (the Gharna Bed) with Chama squam- 

 mosa, Terebratula bisinuata, Terrebellum, sopitum, Valuta 

 costata, V. humerosa, and Conus scabriculus .... 18 



At Alum Bay the Chama Bed is rarely exposed, and the higher 

 beds are represented by a mass of yellow and white sand from 90 

 to 100 feet thick, which has been largely dug for glass-making. 

 These sands are sometimes called the Headon Hill Sands, and at 

 the east end of the island they thicken to 206 feet, and the Chama 

 Bed is seen below them with a thickness of 15 feet. 



C. THE KEGION OF VOLCANIC ACTIVITY 



The northern parts of the British Isles were a scene of 

 remarkable volcanic activity throughout a large portion, if not the 

 whole of Eocene time, and evidences of this activity are found not 

 only in the vast lava -flows above mentioned but in lava -filled 

 fissures of great length. The region over which such phenomena 

 have been found embraces the north of England, the north of 

 Ireland, and the whole of Southern and Western Scotland, and it 

 must also have included the area of the adjoining seas as far south 

 as Anglesey, as far west as St. Kilda, and as far north as the Faroe 

 Islands a total area of about 50,000 square miles. Its history has 

 been written by Sir Archibald Geikie 9 and Mr. A. Barker, 10 and 

 the following is an epitome of it. 



This great development of volcanic action seems to have 

 coincided with a general elevation of the region above indicated, 

 as if it had been pushed upward by the accumulation of a vast sea 



