562 ERUPTIVE AND METAMORPHIC ROCKS. 



of these is supposed to be rich in basic constituents, and may be in part metalHc, 

 while the upper zone is riclier in silica. Nevertheless, many of the chief 

 differences in eruptive rocks are due to the fact that they have solidified under 

 diffei'ent conditions, and they do not debar us from the inference that the two 

 groups of eruptive rocks may often have arisen from the same deep-seated sources. 

 The fact that some masses of native iron resembling meteorites have been found 

 in association vi-ith basaltic rocks in Greenland has been taken to indicate that they 

 might have been derived during volcanic outbursts from the interior of the earth. ^ 



Happily, all rocks may be said to shade one into the other, and 

 it might be possible to collect a series of specimens to show a 

 transition from Granite to a modern lava, while the ultimate 

 chemical composition might not vary very much.'^ The term Trap 

 is an indefinite, and therefore sometimes very convenient, term 

 applied to Eruptive rocks which cannot be distinctly identified in 

 the field. ^ As our sedimentary strata were originally derived from 

 the original crystalline (Igneous) rocks which formed the earliest 

 land, so they contain to some extent similar chemical ingredients, 

 and thus the metamorphism of certain strata may result in molecu- 

 lar changes which would produce a rock similar to one directly of 

 igneous origin.^ Both Eruptive and IMetamorphic Rocks may be 

 of any age ; and there is no material distinction between the 

 older and newer. The old notion that Granite was always the 

 oldest rock has long since been exploded. In the Dartmoor 

 district it is of Post-Carboniferous or Permian age; and in the 

 western islands of Scotland there is granite of Tertiary age. 



Whether any strictly primitive rocks are now exposed at the 

 earth's surface is not known, for the origin of the oldest crystalline 

 schists and granitic rocks is uncertain. Although some of these 

 rocks may be closely related to the upper or siliceous magma, yet 

 in many instances the granitic masses appear to be the ' roots ' or 

 ' cooled reservoirs ' from which volcanoes have in early times been 

 supplied;^ in these cases the granite would be of a more or less 

 intrusive character. Other instances may occur where granitic 

 masses had no direct connection with volcanoes. But the oldest 

 rocks, whether of igneous or aqueous origin, have been so crushed 

 or 'sheared' and altered by mechanical pressure (regional metamor- 

 phism), as well as hydrothermal action, that their origin remains 

 in many cases an enigma ; for both crystalline and sedimentary 

 rocks have in places been converted into schists.® 



^ Rutley, Study of Rocks, 1879, p. 34 ; Sir J. W. Dawson, Address to Brit. 

 Assoc. 18S6 ; Judd, Volcanoes, pp. 312, 318. On the subject of Meteorites, see 

 p. 5 of the present work ; also T. M. Hall, Mineralog. Mag. iii. i ; Dr. W. 

 FHght, G. Mag. 1875, 18S2 ; D. Forbes, G. Mag. 1872, p. 229. 



- See Judd, Volcanoes, pp. 57, 61, 145. 



"* The word Trap was used by Bergmann, from irappa, Swedish for a flight of 

 steps, because many rocks of this class occur in great tabular masses of unequal 

 extent, so as to form a succession of terraces or steps on the sides of hills. Lyell, 

 Elements of Geology, ed 6, p. 587. 



* These rocks are sometimes termed 'Hypozoic' when they occur as fundamental 

 rocks beneath fossiliferous strata. 



* J. C. Ward, Q. J. xxxii. 28; Judd, Volcanoes, p. 145; Bonney, Address to 

 Geol. Soc. 18S6, pp. 87, no. 



® See B. N. Peach, Proc. R. Phys. Soc. Edin. ix. 22. 



