The form and distribution of dolerite masses in the Builth-Llandrindod Inlier, Radnorshire

Abstract

Numerous albite-dolerite masses occur within the inlier of Ordovician rocks which extends between Builth Wells, Breconshire, and Llandrindod Wells, Radnorshire. These are surrounded by Silurian rocks which in many places rest unconformably upon them. Nearly all the dolerites are intruded into shales and show a tendency to occur at nearly constant levels. The lowest horizon belongs to the zone of Didymograptus bifidus , which is succeeded by the zone of D. murchisoni ; this is followed by the Glyptograptus teretiusculus zone and this in turn by the zone of Nemagraptus gracilis and the Dicranograptus shales. Each of these has dolerites at one level, except that in the G. teretiusculus shales there are two levels. It has been found that the largest and most continuous bodies occur at the lowest horizon, and at each successive horizon upwards the dolerite masses become more and more divided, culminating in the remarkable set of intrusions near Llandrindod described in the previous communication. Below the upper G. teretiusculus horizon, the masses appear to be concordant and their feeders are unknown. At and above that horizon many of the masses are very small and grouped in clusters which suggest comparison with the plug-like feeders of the Llandrindod laccolith; they are associated with large oval masses of concordant habit. The mechanism of the intrusion, the probable temperature of the magma and the effect of depth on the form and distribution of the dolerite masses is discussed.