ARCH^AN. 35 



and even in early PaLxozoic times they may have been very 

 great : hence the destruction of land and the formation of 

 sediment may have been comparatively rapid in those days.^ 



ARCHyEAN. 



This term, introduced by J. D. Dana in 1874, is now 

 generally applied to the rocks older than the Cambrian, and 

 which are frequently designated as Pre-Cambrian. These are 

 the oldest rocks exposed at the Earth's surface. Formerly 

 the term Laurentian was applied to the most ancient rocks in 

 the British Islands. 



The term Laurentian was given in 1854 by Sir William E. Logan 

 to a series of highly contorted gneissic rocks which occur in 

 Canada, in the country drained by the St. Lawrence, and which are 

 among the oldest known sedimentary rocks in the world. Their 

 thickness has been estimated at 30,000 feet. Zones of altered 

 limestone included in the series have yielded traces of structure 

 supposed to be foraminiferal, and named Eozoon Canade?ise by Sir 

 J. W. Dawson. It is only right to add that the organic nature of 

 Eozoon has been seriously questioned.^ The occurrence also of 

 Graphite in these rocks has been considered as evidence of 

 vegetation having existed. 



The existence of Laurentian rocks in England and Wales, or o 

 English rocks equivalent to the oldest rocks of America, has not 

 been definitely determined ; nor is it probable that, in comparing 

 the rocks of these two countries, we can ever do more than state 

 that certain series of strata are homotaxial, or in other words that 

 they occupy the same relative position in regard to the succession 

 of life in the two areas. The oldest rocks in North Britain are 

 unconformably overlaid by the Cambrian rocks. To these rocks 

 Blurchison in 1858 applied the term Lewisian or Fundamental 

 Gneiss — the former term being taken from Lewis, the largest 

 island of the Hebrides. 



In several localities in England and Wales there is evi- 

 dence of the presence of rocks of Pre-Cambrian age, and these 

 are now grouped under the general name Archaean. No 

 fossils have as yet been positively identified in any of these 



1 Nature, Dec. 28, 18S2. 



2 An Old Chapter in the Geological Record, by Prof. W. King and Prof. T. 

 H. Rowney, 1881. 



