14 ARCHJSAN TIME. [CHAP. I. 



canic rocks, felspathic lavas and ashes, or of hypocrystalline 

 rocks, including schists of many kinds, quartzites, dolo- 

 mites, and altered grits. 



Some geologists think that the upper group lies un- 

 conformably on the lower, and that they should be regarded 

 as widely separated systems ; others think that the appa- 

 rent unconformity is only a local one connected with the 

 volcanic origin of the newer group ; while some and 

 particularly Dr. A. G-eikie regard the granitoidite as 

 truly igneous rock of later date than the hypocrystalline 

 strata. 



It is obvious that while such diverse opinions exist with 

 regard to the relations of the Archaean rocks, little can be 

 said concerning their probable origin and mode of forma- 

 tion, or concerning the physical conditions of the earth's 

 surface at this early period of its history. Mr. J. E. Marr 

 has even ventured to doubt whether any of the Archaean 

 rocks were originally marine sediments ; he points to the 

 great amount of volcanic activity which is testified by the 

 intrusive and eruptive rocks of the Welsh, Shropshire, and 

 Charnwood districts, and he suggests that even some of 

 the hornblendic and chloritic schistose rocks may only be 

 metamorphosed tuft's. The limestones are thin, lenticular 

 beds, and might, he thinks, have been formed in lakes, or 

 by the calcareous springs which are common in volcanic 

 districts. 



From these and other facts he concludes that Archaean 

 time was one of continued and universal vulcanicity, and 

 that the amount of erosion and denudation which took 

 place at its close was so violent, rapid, and extensive that 

 great masses of rock, which had been metamorphosed at a 

 considerable depth below the original Archaean land, were 

 in many places brought to the surface ; while the greater 

 part of the unaltered surface rocks were swept away and 

 re- deposited as Cambrian sediment. 



