30 
character is resumed. This metamorphic action is due to heat, 
and in these portions of the cretaceous beds all traces of organic 
remains are completely effaced. There seems to have been a 
series of overflows of the trap, indicated by the beds of 
different thickness to he seen in every section. In some cases 
the intervals between the overflows were sufficient to allow the 
surface of the basalt to be disintegrated and made fit for the 
growth of vegetation before another overflow took place. This 
is indicated by the bed of decomposed trap that alternates with 
the solid rock and the beds of lignite or wood-coal which occur 
between the layers of basalt. These circumstances would serve 
to show that forests grew on the surface produced by the disin¬ 
tegration of the early rock, and that this mass of vegetation 
had been destroyed and carbonized by a subsequent flow of 
molten basalt. After speaking of the causes producing the 
different levels of the rock in various parts, Dr. Procter pro¬ 
ceeded to consider the cause of the prismatic structure assumed 
by the columnal basalt in difierent parts of the United King¬ 
dom. The prismatic structure is the result of the contraction 
on cooling of the molten basalt, just as wet clay or starch 
assumes a similar structure on drying. These prisms are 
directed into regular or irregular intervals, the cross fracture 
being nearly at right angles to the prisms, the surfaces of union 
are sometimes quite flat or sometimes curved into a convex or 
concave surface. The explanation of this structure, which is 
generally accepted, was promulgated by Gregory Watt in 1804. 
He observed, during the cooling of basalt fused in a furnace, 
that small globules appeared and increased by the formation of 
external concentric coats like those of an onion, so that even¬ 
tually a number of solid balls were formed, each enveloped in 
several concentric coats to these by pressure, became converted 
into short hexagons. Each ball, resting directly and centri- 
cally on the one below it, forms a column of hexagonal columns 
with cross joints. Dr. Procter then described other theories 
which had been proposed to explain the phenomena, and con¬ 
cluded with some remarks on the relative value of the several 
hypotheses. 
V 
iVIAK 1 ud§ 
