G. H. Morton— Geology of the Me of Man. 2 1 1 



the danger of laying stress upon individual observations which 

 may be taken where the beach was left very thin, or at different 

 parts of its seaward slope. 



The base of the Cornish raised beaches above high- water is shown 

 by observations to average 5 feet : such cases as Pendeen Cove 

 (14) ; Tremearne (9 b) ; Nanjulian (11 c) ; Porth Just (12 a) ; being 

 exceptional. Taking the thickness of the old beaches at 15 feet as a 

 maximum, the average subsidence indicated by them would be from 

 12 to 20 feet below high water. 



De la Beche (Geological Manual, p. 157) gives a section of the 

 successive faces (indicated by dotted lines) that the degradation of a 

 cliff composed of Head upon raised beach would be likely to exhibit 

 (see Fig. 6). 



Sea-level. ,/? 1 5 ^'//i/f///////n/////.'//////fi//////////////^:/n/n'//// l Sea-level. 



Fig. 6. — H, Head, concealing a raised beach, resting upon slate, S. above the 



sea level. 



The raised beach platform has been cut too far back to allow 

 of such cliff faces as 1 and 2. Exceptions to this rule may be 

 furnished by the low tract at Spit Point near Par ; the lowlands of 

 St. Keverne (7 c) ; the flatfish tract covered by blown sand between 

 Constantine and Perleze Bays, if the waterworn sand (in 19 e) is a 

 trace of raised beach, or rests' on an old beach platform ; the gently 

 sloping tract bordering the coast near Trevone (19 g). 



The cliffs bordering a part of Pra Sands are wholly composed 

 of Head to a height of 60 feet from the present beach ; but as Head 

 rests on a portion of raised beach on an adjacent promontory, on a 

 platform 5 feet above high water, the old beach platform may in 

 this instance have been broken up by fluviatile agencies prior to or 

 during the accumulation of the Head ; or the original surface of the 

 platform must have been most irregular. Such cliffs as Nos. 3 and 4 

 are by far the most general sections on the Cornish coast, which has 

 been in very many places cut too far back to show either raised beach 

 or Head. 



{To be continued in our next Number.) 



VI. — Geology of the Isle of Man. 

 By G. H. Morton, F.G.S. 



IN the Geological Magazine for 1877, Dec. II. Vol. IV., pp. 410, 

 456, there is an article on the " Geology of the Isle of Man." 

 by Mr. Henry H. Howorth, to which I desire to call attention. Most 

 geologists are aware that the late Kev. Joseph G. Cumming, M.A., 

 F.G.S., wrote a work entitled " The Isle of Man," in 1848, and that 

 it contains a geological description of the island. In this work, and 



