Hej^orts and Proceedings — Geological Society of London. 141 



group than to the true granites, although it is somowhat more acid 

 than the majority of tonalites. The more bawic types include 

 dolerites, quartz-dolerites, and a rock type intertnediate between 

 quartz-dolerites and granophyres, for which no satisfactory name 

 seems to exist. There is also a development of peculiar rock types 

 as the result of the re-mixing of previously differentiated partial 

 magmas of an acid and a basic character respectively. A study of 

 the distribution of different types of granopliyric structure shows 

 a certain regularity of arrangement, and an attempt is made to 

 reconcile these with known physical laws, especially with reference 

 to eutectics ; and it is concluded that the structure is the result of 

 crystallization imder conditions intermediate between those virhich 

 produce typical plutonic and hypabyssal rocks. 



II.— February 7th, 1906.— J. E. Marr, Sc.D., F.R.S., President, in 

 the Chair. The following communications were read : — 



1. " The Carboniferous Limestone (Avonian) of the Mendip Area 

 (Somerset), with especial reference to the Palasontological Sequence." 

 By Thomas Franklin Sibly, B.Sc, F.G.S. 



The Avonian rocks are exposed in four main anticlinal forms or 

 periclines — those of Black Down, North Hill, Pen Hill, and Beacon 

 Hill ; each of which has an approximately east-and-west trend and 

 has Old Bed Sandstone exposed in its core. The following is the 

 zonal succession : — 



In the present paper, the faunal sequence is discussed in detail, 

 attention being confined almost entirely to the corals and brachio- 

 pods, which predominate throughout the series. The lithological 

 character of each zone and subzone is treated briefly. The general 

 stratigraphy of the area is briefly discussed, reference being made to 

 the more important forms. Following this, the exposures examined 

 are classified in zonal order, and tabulated under the zonal headings. 

 The best exposures of each zone receive special attention. 



