56o ERUPTIVE AND METAMORPHIC ROCKS. 



level was noticed, but there was a marked though temporary change in the level of 

 underground waters in the district most affected by the shock.' J, 



Britain has, during long periods of its geological history, formed portions of a 

 continental area near the sea-margin ; hence it has been liable to volcanic activity 

 and to fluctuations in level.- According to Mr. Godwin-Austen, the coast from 

 Lewes Levels to Chichester Harbour, and on to Hurst Castle, exhibits signs of 

 undergoing elevation at the present day. The coast of the Isle of Wight, opposite, 

 seems, on the contrary, to be suffering depression ; whilst the back of the island 

 exhibits some curious signs of local oscillation. At Sidmouth there is evidence of 

 subsidence now going on at the rate of one inch in ten years. ^ Reference has been 

 previously made to some of the causes of upheaval and depression (see p. 4), the 

 earth's crust being subject not merely to oscillations of a more or less permanent 

 character,^ but to minute vibrations (earth tremors), which appear to be due to 

 fluctuations in barometric pressure.* 



ERUPTIVE AND METAMORPHIC ROCKS. 



During the past twenty years the study of Eruptive rocks has 

 made great progress, and, so far as the rocks in England and Wales 

 are concerned, we are particularly indebted to Mr. S. AUport, Mr. 

 David Forbes, Dr. H. C. Sorby, Mr. J. A. Phillips, Mr. F. Rutley, 

 the Rev. J. Clifton Ward, Prof. T. G. Bonney, Mr. Thomas Davies, 

 Mr. J. J. H. Teall, and Mr. G. A. J. Cole. 



The study requires much special knowledge and training, and it 

 is perhaps attended with a greater amount of minute detail than 

 the ordinary field-geologist can appreciate. For it is desirable to 

 determine not only the many varieties of rock and their com- 

 position, but also the physical condition of their mineral com- 

 ponents : so that microscopic and sometimes also chemical 

 investigations are needed. "^ 



Dr. A. Geikie remarks that the characters by which an eruptive rock may be 

 distinguished are partly lithological and partly structural. Among the more 

 important of them are the predominance of silicates, mica, augite, olivine, etc. ; 

 a prevailing crystalline structure ; the frequent presence of vitreous and devitrified 

 matter, visible macroscopically '' or microscopically ; and the occurrence of 

 por|Dhyritic, cellular, pumiceous, slaggy, and amygdaloidal and fluxion structures. 

 These characters are never all united ia the same rock. On the whole, the most 



1 R. Meldola and W. White, Report on the East Anglian Earthquake, 1885, 

 pp. 183, 209 ; R. Meldola, P. GeoL Assoc, ix. 20 ; H. B. W., Trans. Norfolk. 

 Nat. Soc. iv. 31 ; Kinahan, Proc. Royal Dublin Soc. iv. 318. 



- See J. S. Gardner, G. Mag. 1885, p. 145; Milne, Earthquakes, p. 344; 

 Hughes, Later Movements of Elevation and Depression (Victoria Inst.). 



^ P. O. Hutchinson, in Topley's Report on Erosion of Sea Coasts, Brit. 

 Assoc. 1885. 



* See also Babbage, Proc. G. S. ii. 73 ; and Sir J. W. Dawson, Address Brit. 

 Assoc. 1886. 



^ G. and H. Darwin, Brit. Assoc. 18S1 ; Milne, Earthquakes, p. 310. 



^ Rutley, Study of Rocks, ed. 3, 1884 ; and The Felsitic Lavas of England and 

 Wales (Geol. Survey), 1885 ; J. J. H. Teall, British Petrography (in course of 

 publication) ; Bonney, Addresses to Geol. Soc. 1885, 1S86 ; 1). Forbes, Q. J. 

 xxvii. 311 ; Kinahan, Handy-Book of Rock Names, 1873. The works of Foreign 

 authors must also be studied. 



'' The term is applied to characters visible by the eye in hand-specimens. 



