IxXXvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The first I shall advert to are the proofs he brings forward of ex- 

 tensive volcanic action throughout the whole series of the palaeozoic 

 rocks. There is evidence, he tells us, "that during the period when 

 the Llandeilo flags and their equivalents in the Silurian system were 

 accumulated over the area extending from Malvern to Pembrokeshire, 

 volcanic points existed from whence molten matter and often ashes 

 were ejected, and were intermingled with the detrital accumulations of 

 the period;" that "trappean ash, the volcanic ash of the period, 

 was mingled with the gravels and sands now forming conglomerates 

 and sandstones, that it was accumulated in beds, interstratified with 

 mud and sand, and that the remains of the crustaceans of the day are 

 found in it." This ash, moreover, he is of opinion points to sub- 

 aerial volcanos, and probably therefore to land from which it may have 

 been carried within a moderate distance*. This ash often covers 

 an area of great extent. The facts of the contemporaneous existence 

 and interstratification of igneous rock with the fossiliferous and in- 

 ferior slates had been before pointed out by Professor Sedgwick and 

 Sir R. Murchison, and the latter in his ^ Silurian System ' frequently 

 adverts to the intermingling of volcanic ashes ; but, if I am not mis- 

 taken. Sir H. De la Beche is the first who has pointed out the probable 

 existence of sub-aerial volcanos at that remote period of the earth's 

 history ; that is, active volcanos on land, prior to the eruption, as I 

 shall presently mention, of the granite of that region, which at no 

 distant period geologists were accustomed to consider as the deep-laid 

 foundation on which all the superincumbent slates rested ; on which 

 slates also the venerable name of primitive was conferred. 



Ascending in the series, he describes Devonian strata near Tavistock, 

 argillaceous slate and limestone intermingled with fused trap and ashes, 

 a pumice filled with carbonate of lime, and the remains of mollusks in 

 ashes ; and much ash and vesicular igneous rocks are intermingled, 

 he says, with the beds of the South Devon limestones f. 



Treating of the carboniferous series, he describes Brent Tor as pre- 

 senting a mixture of trap rocks, ash, and a conglomerate containing 

 vesicular portions of igneous rocks, which approach the condition of 

 pumice ; and adds that " these rocks are associated in a manner such 

 as is often seen in volcanic countries |." 



The author is of opinion that the protrusion of the granite of Corn- 

 wall and Devon clearly took place after the deposit of the coal-measures 

 of Devon, and anterior to that of the New red sandstone series. " The 

 Devon and Cornish granites," he says, " seem to have been thrust up 

 through points of least resistance, in a line extending from the southern 

 part of Devonshire to the Scilly Islands,part having protruded through 

 the weakest places, and the remainder still concealed beneath, suppo- 

 sing this granite connected below, at moderate depths, during the whole 

 distance§." — "From the Scilly Islands to Dartmoor inclusive, we seem 

 to have the up thrust of one mass, which found points of less resistance 

 amid the superincumbent accumulations more in some places than in 

 others. As the masses rose, the edges of the detrital, trappean and 



* Pages 30 to 35= f Pages 84 to 90, X Page 137. § Page 228. 



