GEOLOGICAL NOTES ON THE ISLE OF PORTLAND. 59 



of ordinary spring tides at the Portland end, was ascertained by 

 Sir John Coode to be 200 yards, and its height 42 feet 9 inches, 

 and at the Abbotsbury end 170 yards wide and 22 feet 9 inches 

 high. It extends to a depth of 48 feet below high water at 

 Portland, and only 30 feet at Abbotsbury, where the beach joins 

 the shore ; at Burton-Bradstock cliff its depth below the sea- 

 level decreases to 21 feet, and to 9 feet at Bridport Harbour. 

 Sir John Coode describes the Bank as composed chiefly of chalk- 

 flints, with a small proportion of pebbles from the Eed Sand- 

 stone, some are of a dull red colour others brown or dark yellow, 

 with occasional red marks resembling blood spots ; a peculiar 

 kind of jasper pebbles, with flesh-coloured red predominating, are 

 not very uncommon. These have been sometimes mistaken for 

 Devonian limestone, but they do not contain any calcareous 

 matter, as there is no effervescency on the application of 

 muriatic acid. There are also occasionally, pebbles which are 

 decidedly porphyritic, both green and red ; these are compara- 

 tively rare, but found in sufficient numbers to prove that thtir 

 presence is due to something more than accidental causes. These 

 materials are not derived from the beds of the neighbourhood ; 

 for the rocks between Portland and Lyme Eegis are Jurassic ; 

 beyond Lyme and as far as Sidmouth, beds of chalk with schists 

 cap the new Eed Sandstone cliffs, which extend westward 

 past Budleigh Salterton, where the beach is almost entirely com- 

 posed of pebbles of precisely the same kind as those of the 

 Chesil Bank. The jaspar pebbles are traced to Aylesbere, six 

 miles inland ; the pebbles of porphyry are referred to the Heavi- 

 tree Conglomerates, which are either Permian or Trias, or 

 to similar beds which crop out on the coast between Beer and 

 Torbay. The size of the pebbles increases from Abbotsbury to 

 Portland, where they are flat, ovoid in shape, and from three 

 to four inches across. Sir John Coode suggests that the reason 

 why the large shingle is always found to leeward " Is that as 

 a rule, the large pebbles move more rapidly than the small, 

 because more exposed to the action of the waves." Sir Charles 



