352 SECTIONAL TRANSACTIONS.— C. 



Monday, August 9. 



9. Dr. G. W. Tyrrell and Dr. M. A. Peacock. — Lava Structures in 

 Iceland. 



Structures of the Palagonite Series : 

 Globular basalts. 

 Shattered basalts. 

 Pseudo-agglomerate. 

 Agglutinate, cf. Vesuvius. 



Recent Lava structures : 



HeUuhraun (= ropy lava) and 



Apalhraun (= block lava). 



Lava tumuli. 



Jointing of ' grey basalt ' helluhrauns. 



Craters, Eldborg, Frostadarvatn, etc. 



Fracture Structures : 

 The Allmannagja, etc. 



10. Dr. A. K. Wells. — Variolites from Merionethshire. 



Some igneous rocks exhibiting pillow-structure and occurring in the Ordovician 

 east of Rhobell Fawr, Merionethshire, have proved to be basalts in a better state of 

 preservation than any other material from this region. 



A series of sections cut from a pillow has enabled the course of crystallisation to be 

 followed, there being a complete passage from devitrified glass through variolites to 

 normal sub-ophitic dolerite. 



The rocks were described, comparisons made with kindred types, and matters 

 connected with affinities and nomenclature were discussed. 



11. Prof. Kendall, F.R.S., and Prof. A. Gilligan. — Underclays and 



the ' Growth in Place ' theory of Coal Formation. 



12. Mr. F. B. A. Welch. — The Geological Structure of the Central Mendips, 



The Central Mendips comprise a rectangular area roughly measuring eighty square 

 miles, between the townships of Shepton MaUet and Cheddar on the east and west 

 respectively. 



The area has been mapped on the six-inch scale, applying Vaughan's zones, for 

 the purpose of investigating the tectonics. 



The Mendips consist of a ridge of Palaeozoic rocks running W.N.W. — E.S.E. 

 The structure is usually considered as a simple anticline ; but actually it consists of 

 four major perichnes arranged en echelon, whilst overfolding and thrust-faulting have- 

 played an important part in determining the present distribution of the beds. 



In the Central Mendips the main hiU-mass consists of the North Hill, Pen Hill,, 

 and Beacon Hill pericHnes, having cores of Old Red Sandstone age, with the Carboni- 

 ferous Limestone Series succeeding. 



North of Pen Hill is a large inverted block of beds, showing extensive overthrusting^ 

 which extends north to Emborough. Against the main hiU-mass, on the south side, 

 a large syncline is thrust, extending from Cheddar to Wells. At one point a ' window ' 

 occurs, revealing beds of the main hiU-mass beneath the overthrust. A second parallel 

 overthrust occurs at Ebbor, and remnants of this are seen in small hills north of Wells. 



The inliers of Carboniferous Limestone age in the plain to the south point to con- 

 siderable disturbance ; but, owing to the covering of Triassic strata, relationship to 

 the main hill-mass is not clear. 



Earth movements appear first to have acted from the south, causing a more or 

 less east-to-west ridge. Pressure from the west followed, and buckled the previously 

 formed ridge, the main disturbance being in the Pen Hill region. This, together with 

 differential movement, due to relative resistance of the blocks, has caused the present 

 distribution of beds. 



