66 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



defined and imperfectly known portion of the geological record which 

 underlies the Cambrian system. But it is obviously too indefinite 

 for purposes of precise stratigraphical classification. It unites a 

 vast succession of rocks differing widely from each other in struc- 

 ture and age, and possessing, indeed, only the one common character 

 of being older than the Cambrian age. To class together, for example, 

 under one common name, the massive gneisses of the north-west of 

 Scotland and the well-bedded tuffs and grits below the Llanberis 

 Slates, is as misleading as it would be to put the Llanberis Slates and 

 the Stonesfield Slate together, because they are both pre-Cretaceous. 



We need a more definite series of names for the rock-groups that 

 underlie our Palaeozoic formations, or at least we should agree to 

 apply more precisely the names already in use. " Pre-Cambrian " 

 and " xlrchaean," for example, are often employed as synonymous 

 terms. The former word may be retained as a convenient adjective, 

 to denote that the rocks to which it is applied are older than Cam- 

 brian time. But we do not require two epithets of equal vagueness. 

 It would, in my opinion, be greatly conducive to clearness of defi- 

 nition if we resolved to restrict the term '' Archaean " to the most 

 ancient gneisses and their accompaniments. No method has yet 

 been devised whereby the oldest gneiss of one country can be shown 

 to be the true stratigraphical equivalent of the oldest gneiss of 

 another. Palaeontology is here of no avail, and Petrography has 

 not yet provided us with such a genetic scheme as will enable us to 

 make use of minerals and rock -structures as we do of fossils in the 

 determination of geological horizons. All that can be positively 

 affirmed regarding the stratigraphical relations of the rocks to which 

 I would confine the term " Archaean " is that they are vastly more 

 ancient than the oldest sedimentary and fossiliferous formations in 

 each country where they are found. Although, taken as a whole, 

 they present a remarkable sameness of petrographical characters in 

 all quarters of the globe, we cannot tell whether, for example, the 

 Lewisian gneiss of Scotland is the true equivalent of the Laurentian 

 gneiss of Canada ; undoubtedly they occup}' similar stratigraphical po- 

 sitions, and present a close resemblance in petrographical characters. 



From the very nature of the case, the name by which we designate 

 these ancient rocks cannot possess the precise stratigraphical value 

 of the terms applied to the fossiliferous formations. At the same 

 time, I should guard against the implication that the name is given 

 to a special petrographical type only found in the most ancient 

 gneisses. When the area of observation is sufficiently extensive, we 



