Reports and Proceedings — Geological Society of London. 331 



lacking in ferromagnesian minerals, occurring in dykes and lava- 

 flows. A large number of trachytoid phonolites occur in the form 

 of interbedded sheets ; they are divided by the author into five 

 principal varieties, one of which bears leucite. Nephelinitoid 

 phonolite is rare. A considerable number of rocks are classified as 

 trachydolerites apparently occurring as lavas. The andesites are 

 characterised by hornblende and augite, with little or no olivine ; 

 basanite is restricted in its occurrence. Dolerites of two principal 

 types occur in dykes, one type being the commonest of all the rocks 

 in the area. There are many lava-flows and some dykes of basalt ; 

 and one melilite basalt is found in the extreme north of the district. 

 A note is appended on the economic application of these rocks. 



A considerable series of chemical analyses follows, showing that 

 the silica percentage varies from 66 in the Portobello trachyte to 

 44-84: in one of the dolerites. The petrographical differences are 

 coincident with the chemical differences, and the rocks are generally 

 similar to rocks of the same classes in other parts of the world, 

 and more particularly to those of East Africa. A classification 

 is attempted, according to the methods proposed by Pirsson, 

 Washington, and Iddings ; but the author concludes that, in his 

 case, this classification, while grouping the rocks according to their 

 chemical relationship, is unsatisfactory from a geological standpoint. 

 The chemical composition of the rocks is further represented- in 

 diagrams constructed according to the method employed by Hobbs. 

 These diagrams show clearly the chemical diiferences between the 

 alkaline and the basic series, and the extent to which this hiatus 

 is bridged over by the trachydolerite and basanite. The series, 

 however, does not show a gradation similar to that described by 

 Professor Brogger in the Christiania rocks. 



In the concluding section of the paper the relative ages of the 

 volcanic rocks are worked out, so far as possible. The order does 

 not appear to be explicable on any theory of magmatic differentiation, 

 but it is suggested that it may be due to the mixing of magmas 

 before and during eruption. 



2. " The Carboniferous Limestone of the Weston-super-Mare 

 District." By Thomas Franklin Sibly, B.Sc. (Communicated by 

 Dr. A. Vaughan, B.A., F.G.S.) 



The Carboniferous Limestone of the Weston Worle Eidge includes 

 the greater part of the Syringothyris zone (C), extending from the 

 * laminosa dolomites ' upwards, and a large part of the Seminula 

 zone (S). While the dip of the rocks of the ridge is towards the 

 south, a reversed fault throws the Syringothyris Beds on the south 

 against the Seminula Beds to the north, and the latter rocks are 

 overfolded on the north side of the fault. To the south of the 

 fault there is a sequence from Ci to 83 ; to the north, S only is 

 represented. The lower part of Ci, consisting of the ' laminosa 

 dolomites ' and the ' Caninia Oolite,' resembles the equivalent part of 

 the Clevedon sequence, and indicates shallow-water conditions ; 

 the upper part of C, consisting of a thick mass of fossiliferous 

 limestone, closely resembles the corresponding part of the Burrington 



