346 PKOCKEDIWGS OF THE GEOLOGICAL SOCIETY. [Mar. 7, 



rest of the area clear water prevailed, in which limestone was formed 

 from the Crinoids and other animals that flourished in that part. 



15. Sudden Changes from thiclc Limestone to Mechanically formed 

 Rocks. — Many persons may feel a difficulty in supposing it possible 

 that a sudden change can take place from thick limestones to mere 

 slates and sandstones. My own experience among coral-reefs, how- 

 ever, relieves me from any such difficulty. The barrier-reefs of the 

 north-east coast of Australia, for instance, are continuous for about 

 1200 statute miles, and have a steep edge throughout that course, 

 which in one part at least is upwards of 1800 feet in depth. They end 

 against the coast of New Guinea, on the north side of Torres Straits, 

 in a massive reef, called the Warrior Eeef, which is itself thirty miles 

 long and as massive, and steep too, though by no means so deep, as 

 any other reef to the southward. Immediately east of it, however, 

 the coast of New Guinea is fronted by extensive mud-flats ; and 

 shoal water, with a muddy and sandy bottom, stretches oif the coast 

 for sixty miles from the shore ; and the land consists of mangrove- 

 swamps for an unknown distance into the interior, and this for a 

 space of 150 or 200 miles along the coast. There must then be as 

 abrupt a change here as we can imagine, from a widely spread and 

 continuous calcareoas mass, to one consisting principally or entirely 

 of a mechanically formed deposit. 



No one can study the Carboniferous Limestone minutely without 

 seeing that it is essentially composed of crinoidal fragments. Even 

 in the compact parts of the limestone, examination with the lens will 

 disclose many little shining faces of crystalline Calcite with a dot in 

 the middle, which Professor John Phillips long ago pointed out to 

 me as a sure indication of a crinoidal joint. The limestone has been 

 formed by the growth of submarine forests of Encrinites ; the debris 

 of each successive generation being ground down or decomposed into 

 calcareous mud. So far as the circumstances of depth and bottom 

 favoured the growth of these animals, they flourished ; where there 

 occurred a sudden change in those circumstances, they ceased to grow. 

 The very incoming of quantities of mud and sand would be one of 

 the circumstances most likely to arrest their growth. The animals 

 inhabiting seas with a sandy or muddy bottom would naturally be 

 somewhat different from those hving in the clear water among the 

 forests of Crinoids. Neither would it appear to me at all surprising 

 if, when circumstances favourable to the growth and formation of 

 local banks of limestone occurred within the muddy and sandy area, 

 some animals should live there of a diff'erent species or genus from 

 those in the other part of the sea where the crinoidal forests grew. 



III. Geological Steuctijee of Nokth Devon. 



1. Baggy Point to Dulverton. — In a paper read before the Dublin 

 Geological Society* on May 10th, 1865, and published in their 



* The name of this Society has lately been altered by Her Majesty's per- 

 mission into that of the Royal Geological Society of Ireland, the members being 

 empowered to call themselves Fellows thereof. 



