Geological Society. 447 



Since an irregular land-surface was thus depressed beneath the 

 sea, one might expect that in some of the deeper valleys deposits 

 older than the submergence might be detected. Also, seeing that 

 the land has slowly risen again many hundred feet, we ought to 

 have evidence of the subaerial denudation which has been going on 

 since the land began to rise. 



Thus the author starts with the definition of three important 

 ages of long duration, and proceeds to refer some of the best exposed 

 gravel-deposits of East Anglia to one or other of them. 



They are, in descending order : — 



(A) The stage of which the Barnwell Gravel is taken as a type. 



(B) The stage of which the Plateau Gravel is the most important 



representative. 



(C) The stage to which he suggests that the Barrington Beds may- 



belong. 



The rest of the paper consists of descriptions of sections, and 

 discussion of evidence derived from fossil remains. 



2. ' The Pitchstones of Mull and their Genesis.' By Ernest 

 Masson Anderson, B.Sc, M.A., F.G.S.. and E. Gr. Radley. 



The pitchstones here discussed occur with extraordinary frequency, 

 intruded into the Tertiary plateau-lavas of the eastern portion of 

 the Ross of Mull, as well as in less number in other parts of the 

 island. 



They fall into two main divisions, distinguished by the absence 

 or by the presence of porphyritic felspars. Those of the non- 

 porphyritic class are the most prevalent, and usually form the 

 central portion of sills or inclined sheets. The marginal portion of 

 these intrusions is crystalline or stony. The penological characters 

 of these pitchstones. and their more crystalline margins, are such 

 that they seem to warrant the grouping of the rocks under a new 

 type-name, and the name leidleite has been chosen. The por- 

 phyritic pitchstones occur as tlat or gently-inclined sheets ; they 

 also are associated with a more crystalline phase, and have been 

 grouped under the type-name inninmorite. 



The relation of the stony margins of the pitchstone-intrusimis 

 to their glassy centres is usually seen clearly. A typical leidleite 

 may have 5 feet of pitchstone in the centre, with margins of 

 stony matter 3 feet thick on each side. Occasionally, the central 

 glassy portion may be split up by stony partings. 



A feature that occurs very frequently in these rocks is what 

 has been termed ' sheath-and-core ' structure. In this case, the 

 stony base and top of an intrusion send off narrow sheets of stony 

 character which traverse the glassy portion in a branching and 

 sinuous manner. The glassy nature of the cores is clearly not 

 due to a greater rapidity of cooling ; but, with the object of ascer- 

 taining the reason for the devitrification, a chemical investigation 

 of both the glassy and the stony portions was undertaken. 



