In his Geological Maps of Devon and Cornwall, which are to the present time those issued by the Geological Survey, De la Beche made no attempt to discriminate between the varieties of igneous rocks, save that the basic "greenstones" were distinguished from the acid bosses of granite and the elvans. But in his classic "Report" much more detail was inserted, showing that he clearly recognized the existence both of volcanic ashes and of lavas, as well as of intrusive sheets. At the outset of his account of the "Grauwacke," he remarks that the sedimentary deposits are accompanied with igneous products, "a portion of which may also be termed sedimentary, inasmuch as it would seem to have been deposited in beds among contemporaneous rocks of the former description by the agency of water, after having been ejected from fissures or craters in the shape of ashes and cinders, precisely as we may now expect would happen with the ashes and cinders ejected from volcanoes, particularly insular and littoral volcanoes, into the sea."[296] Again he speaks of "two kinds of trappean rocks having probably been erupted, one in the state of igneous fusion, and the other in that of ash, during the time that the mud, now forming slates, was deposited, the mixtures of volcanic and sedimentary materials being irregular from the irregular action of the respective causes which produced them; so that though the one may have been derived from igneous action, and the other from the ordinary abrasion of pre-existing solid rocks, they were geologically contemporaneous."[297] He recognized the origin of the amygdaloidal varieties of rock, and by dissolving out the calcite from their cells showed how close was their resemblance to modern pumice.[298]
[296] "Report on the Geology of Cornwall, Devon and West Somerset," Mem. Geol. Survey, 1839, p. 37.
[297] Op. cit. p. 57.
[298] Op. cit. pp. 57, 61.
Since these early researches many geologists have studied the igneous rocks of Devonshire. I would especially refer to the labours of Mr. Allport,[299] the late J. A. Phillips,[300] Mr. Rutley,[301] the late Mr. Champernowne,[302] Mr. W. A. E. Ussher,[303] Mr. Hobson,[304] and General M'Mahon.[305] Mr. Champernowne in particular has shown the abundance of volcanic material among the rocks of Devonshire, and the resemblance which in this respect they offer to the Devonian system of North Germany.
[299] Quart. Journ. Geol. Soc. xxxii. (1876), p. 418.
[300] Op. cit. xxxi. (1875) p. 325, xxxii. (1876) p. 155, xxxiv. (1878) p. 471.
[301] "Brent Tor," Mem. Geol. Surv. p. 18; Quart. Journ. Geol. Soc. lii. (1896), p. 66.
[302] See in particular his last paper "On the Ashprington Volcanic Series of South Devon," Quart. Journ. Geol. Soc. vol. xlv. (1889), p. 369.
[303] This geologist has spent many laborious years in the investigation of the geology of Devonshire, and has published numerous papers on the subject, in the Transactions of the Devonshire Association and of the Royal Cornwall Geological Society, in the Proceedings of the Somersetshire Archæological and Natural History Society, and of the Geologists' Association, in the Geological Magazine, and the Quarterly Journal of the Geological Society. Reference may especially be made to his Memoir in the last named journal, vol. xlvi. (1890), p. 487.