192 Geological Society. 



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 differences 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 

 Prof. 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 differentia- 

 tion, but it is suggested that it may be clue 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. 



The Carboniferous Limestone of the Weston- Worle Ridge includes 

 the greater part of the Syrinc/othyris-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 SyrhujotJiyris-Beds on the south 

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

 over-folded on the north side of the fault. To the south of the 

 fault there is a sequence from C x to S 2 ; to the north, S only is 

 represented. The lower part of C l5 consisting of the ' laminosa- 

 dolomites ' and the ' CVmmm-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 Bur- 

 rington section, and indicates the predominance of a Mendip-facies. 

 Much dolomite occurs in the lower part of S, indicating shallow- 

 water conditions during part of Seminula-time. There is a distinct 

 development of the semireticidatus-suhzone (S x ), which exhibits a 

 faunal overlap between C and S; and the base of 8 2 is remarkable 

 for its rich and varied fauna. 



The Woodspring Ridge shows a sequence from the Upper 

 Zaphrentis-Zone (Z.,) to the top of C 1 , the whole sequence being 

 exactly similar to "that of Clevedon. In this ridge the contem- 

 poraneous igneous rocks occur in horizon y. In the Weston- 

 Worle Ridge they are put above the ' Caninia-Oolite' that is, about 

 450 feet below the top of C. It is, therefore, evident that there were 

 two periods of volcanic activity, one of which occurred at the close 

 of Z/tjihrentis-time and the other early in Syrhujothyris-time. Notes 

 are given on the faunal sequence. 



