318 Geological Society. 
stones of much greater thickness than in any other part of their 
course. Mr. Murchison has also shown that rocks oceupying a large 
coast tract in Pembrokeshire, which from their mineral aspect had 
been laid down as “greywacke”, consist of true coal-measures. 
After noticing a ridge of intrusive rocks in Caermarthenshire, be- 
tween the Towey and the Taf, as connected with certain great lines 
of dislocation, he points out, in the Cambrian System of Pembroke- 
shire, examples of the existence of two classes of trap rock, one 
bedded or contemporaneous, the other amorphous and of posterior 
intrusion. He further shows that the main directions of the stra- 
tified deposits of this county are parallel to divergent zones of 
trap. 
In another paper the same author states that he has lately disco- 
vered to the north-west of Shrewsbury, proofs of an eruption of 
trap posterior to the new red sandstone, and probably to the lias. 
This line of fissure along which he has observed the new red sand- 
stone affected for a distance of thirty miles is on the precise pro- 
longation of a linear eruption in the Breiddin Hills, which he had 
previously pointed out as having been in progress during and after 
the epoch of the deposition of the Silurian strata. The more modern 
trap is made up of a peculiar felspathic rock identical with some 
of these at the great vent of eruption fifteen miles distant, where 
they both alternate with and are intruded into the more ancient 
deposits. 
It appears from these observations that volcanic operations were 
renewed along the same line after a wide interval of time, showing 
that we must be on our guard against inferring the synchronism of 
coincident lines of derangement. The repetition also in the same 
spot and at two distant periods of a trap identical in mineral cha- 
racter is curious, and reminds me of an opinion lately mentioned to 
me by Mr. Von Buch, that the composition of lava is often deter- 
mined by that of preexisting volcanic rocks near the point of erup- 
tion. Thus on two opposite sides of the same volcano, as on Tene- 
riffe for example, a trachytic flow of lava will issue from a mass of 
trachyte, and a basaltic flow from rocks of basalt. 
Mr. De la Beche has shown that the trappean rocks are associated 
in such a manner with the new red sandstone of part of Devonshire,— 
among other places, near Tiverton and Exeter,—as to indicate that the 
trap and the sandstone were each in the course of formation at the 
same period. Some beds of sand present every appearance of hav- 
ing been of volcanic origin, and ejected from a crater, but the sand 
became mixed with common detrital matter then in process of de~ 
position at the bottom of the sea. Numerous angular fragments, 
some of them even one or two tons in weight, of quartziferous por- 
phyry with a felspathie base, are intermingled with the conglome~ 
rates of the old red sandstone, and do not resemble any trappean 
rocks discovered in place in this district. Mr. De la Beche conjec- 
tures with much probability that these fragments were ejected from 
volcanic vents, and that they fell upon the sand and pebbles then in 
the course of deposition around such vents, and were thus included. 
The author has not failed to show that the original features of the 
