284 Correspondence — Rev. H. G. Baxj. 



The diminution of the island has taken place almost entirely from 

 one direction, the sea having encroached 30 miles on the N.E. side, 

 and 1 mile only on the S.W. ; this is probably owing to the south- 

 westerly set of the current, and to the harder (? volcanic) rocks 

 forming that portion which now remains as an island. 



A series of soundings over the N.E. area would be valuable, as 

 showing the form of surface presented by this modern "plain of 

 marine denudation." W. H. Penning. 



APPABENT AND TETJE DIP.i 



Sir, — The wording of Mr. Penning' s ingenious paper involves 

 one or two unfortunate slips of the pen. 



In the first rule, instead of " the number of degrees of dip," write 

 " the tangent of the angle of apparent dip." 



In the second rule, proceed as before; but measure the length 

 along one of the lines produced backwards. 



The truth of the principle so modified is obvious from the follow- 

 ing considerations. 



If, along the line h, the rise is 1 in 20 ; and, along the line a, 1 in 

 10 : then, taking 20 units along one line, and 10 along the other, we 

 arrive at the horizontal line in the plane, i.e. at the line of strike. 

 St. Leonard's House, Ludlow. Henry George Day. 



PHYSICAL GEOLOGY OF EAST ANGLIA IN THE GLACIAL EPOCH. 



Sir, — When the abstract of Mr. Penning's paper, " On the 

 Physical Geology of East Anglia during the Glacial Period,^' read 

 before the Geological Society in December last, with the report of 

 the discussion thereon, appeared, I addressed the following letter to 

 the author, in the hope that so much of it as consisted of an ex- 

 planation of the views I held upon the subject of the East Anglian 

 valley system might be allowed to be printed at the end of his 

 paper. This, I believe, Mr. Penning was willing should be done, 

 but the Council of the Society refused to allow it. I therefore, in 

 order to prevent further misapprehension of my views, ask of you 

 the favour of giving the letter a place in your columns. 



Searles V. Wood, jun. 

 [Copy letter.] 



•'•' Dear Sir, — So far as the abstract of it aifords information, the 

 principal fact brought to n.otice in your paper, ' On the Physical 

 Geology of East Anglia during the Glacial Period,' is that the 

 Middle Glacial gravels do not generally range in East Anglia above 

 an altitude of 300 feet, that they are at about this height overlapped 

 on the south side of the Cambridgeshire Chalk escarpment by the 

 Boulder-clay, which ranges to the top of that escarpment, resting 

 on the older formations; and that this Boulder-clay recurs on the 

 opposite side of the escarpment, similarly resting on the older rocks, 

 without any Middle Glacial beneath it. 



"You will find that this limit of the elevation of the Middle Glacial 

 in East Anglia is expressly i)ointed out by me in a paper in the 

 1 See Geol. Mag. for May last, p. 236. 



