36 ARCH^AN. 



rocks. Their position is determined by their relation to the 

 overlying Cambrian rocks, the basement beds of which are 

 often conglomeratic, and contain pebbles of the older rocks. ^ 



The rocks regarded as Archaean consist chiefly of schists, 

 gneiss, quartzose and granitoid rocks, volcanic agglomerates, 

 breccias, and slates. Some unconformities have been recog- 

 nized in the series, but as these rocks were, to a large extent, 

 volcanic, and perhaps also terrestrial, the unconformities may 

 not mark any great break in time.^ The earliest rocks of 

 this period are of a crystalline nature, with veins of eruptive 

 material, which apparently did not find vent at the surface. 

 The rocks have undergone much alteration, and have in 

 some instances been so crumpled, folded, and faulted, that 

 older rocks have been driven over newer rocks by 'overthrust' 

 faults ; while in other instances the rocks themselves have 

 been crushed, drawn out and almost reconstructed in a miner- 

 alogical sense by a process of ' shearing.' 



Little can be said of the physical conditions at this re- 

 mote epoch, but evidences of possible Glacial action in Pre- 

 Cambrian times have been indicated.^ 



The determination of Archaean rocks in England and 

 Wales is mainly due to the researches of Dr. H. Hicks, 

 Prof. T. McK. Hughes, Dr. C. Callaway, and Prof. T. G. 

 Bonney. Some portions of the evidence have, however, 

 been controverted by Dr. A. Geikie, and it will therefore 

 be desirable to point out the differences of opinion which 

 have been expressed respecting the areas in England and 

 Wales where these, the oldest known rocks, have been 

 identified.'* 



Malvern. — The old gneissic rocks and crystalline schists of 

 Malvern, originally considered to be altered Cambrian strata, were 

 in 1863 described by Dr. H. B. HoU as relics of an old Pre- 

 Cambrian continent.^ These rocks, which help to form the tiny 

 mountain range or " metamorphic ridge" of Malvern, rise up in 

 the Herefordshire and Worcestershire Beacons, and in the North 

 Hill." (See Fig. 4.) The rocks, which are traversed by intrusive 



^ See paper on " How to Work in the Archaean Rocks," by C. Callaway, G. 

 Mag. 1881, p. 348, etc. 



- See J. E. Marr, G. Mag. 1883, p. 263. 



■' H. Hicks, G. Mag. 1880, p. 490. 



* See Prof. T. G. Bonney, On the Archcean Rocks of Great Britain, Brit. 

 Assoc. 1885. 



•^ Q. J. xxi. 72 ; G. Mag. 1865, p. 573. 



'■' L. Horner, T. G. S. i. 281 ; Rev. J. H. Timins, Q. J. xxiii. 352 ; J. 

 Phillips, Mem. Geol. Survey, ii. Part i ; W. S. Symonds, Records of the Rocks, 

 P-33- 



