Quarrying activity adjacent to the well-known intrusion of monchiquite cutting Old Red Sandstone near Usk, in Monmouthshire (originally described by W. S. Boulton, in 1911), shows that the monchiquite proper is associated with an agglomerate-filled vent. Fossiliferous blocks contained within the agglomerate (first recorded by C. R. K. Blundell) show that the volcanic activity took place not earlier than in Lower Carboniferous time; and throw light on the former extension of the Carboniferous Limestone in this area.
In the search for large deposits of haematite and other feebly magnetic minerals, it is suggested that mapping the magnetic residuals may be of greater value than mere linear traverses. Not only is the method more suitable for exploring virgin areas, but it also affords a means of differentiating between the effects of large deeply buried masses and small objects at shallow depths. At the same time, the graphical expression of the results on the map becomes more easily intelligible.The application of the method to known haematite ore-bodies in South Wales has yielded striking results and the extension of the survey into an unexplored area indicates the possibility of proving at least one further large haematite deposit.
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