COAL-FIELDS OF SOUTH AFRICA. 673 
that the evidence brought forward indicated a subaqueous origin 
for the trap. Horizontal intrusions are very common in sandstones 
and coal, and, as a general rule, very slight alteration is produced by 
them. 
Dr. Exton said, that the configuration of the country favoured 
the idea that the same lacustrine conditions had formerly existed in 
the south as were now present in central Africa. He called attention 
to a prognostication by Mr. Bain, that a great volcano would be 
found to have existed between the Orange and Vaal rivers, the 
evidence being the trap which remained on the summits of the 
table-mountains in that region. The Diamond-mines are now worked 
within the old volcanic chimneys. Dr. Exton further called 
attention to Mr. Stow’s observations on the relative distribution of 
red beds and coal-bearing rocks, and to his belief that coal, which 
was first found in the south-east of the plateau, would also crop out 
in the nortn-west—a view since confirmed. Coal has been worked 
in the Orange Free State; but the carriage is too costly. The 
author’s description of the country as one vast coal-field was an 
exaggeration. 
