Abstract

The President thanked Dr Brooks for this further contribution to data bearing on the structure of the Bristol Channel basin. He expressed surprise at the similarity of energy propagation velocities quoted for the basal Lias (limestones with at least equal parts of shale) and for the Carboniferous Limestone, presumed to be developed as a massive carbonate, and asked if there was some local explanation.

Dr Brooks replied that although the lower part of the lower Lias of the Vale of Glamorgan contains about equal parts of limestone and shale, there are sequences (notably in the bucklandi Zone) in which the proportion of limestone approaches 80% of the total thickness and within which individual massive limestone bands may be up to a few metres thick. Recent land refraction investigations west of Aberthaw produced a lower Lias velocity of 4.05 ± 0.15 km/s (provisional value). This value lies close to assumed Carboniferous Limestone velocities encountered in the present surveys and draws attention to the problem of assessing the geological significance of refractor velocities in the range from 4.00 km/s to 4.50 km/s.

D. R. Tappin commented on I.G.S. Borehole 72/60 (Brooks & James fig. 5) from which a Tremadocian age has been obtained. The core is 2.5 m in length and is vertically cleaved. It was recovered in a shattered condition beneath 5 m of sand and gravel and 3 m of stiff, brown, stoneless clay. The rock is dark grey, thinly laminated and highly cleaved. Microscopic examination (K. S. Siddiqui personal

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