THE 



GEOLOGICAL MAGAZINE, 



NEW SERIES. DECADE II. VOL. VI. 



No. X.— OCTOBER, 1879. 



OIR-ia-IIIIsr.A.IIL ARTICLES. 



I. — On the Classification of the British Pre-Cambrian Eocks. 1 

 By Henry Hicks, M.D., F.G.S. 



THE results obtained of late years, through the numerous in- 

 vestigations which have been carried on amongst the Pre- 

 Cambrian rocks in Great Britain, combined with the important 

 discoveries of many additional areas to those previously known, 

 seem to require that some general attempt should now be made to 

 correlate the principal groups where they have been observed ; 

 and to classify them according to the prevailing types of rocks 

 recognized therein, and peculiarities observable. 



It will be seen that the ordinary methods of investigation, used in 

 determining the order of succession in more recent strata, are equally 

 applicable to these ; provided, we grant, that they were deposited 

 more or less in a similar manner, and that the crystalline or semi- 

 crystalline state in which they are now found has been subsequently 

 induced. 



This being allowed, we arrive at the probable conclusion that the 

 sediments which hold the lowest, and hence the earliest position, 

 are those which have undergone most change ; and that this will be 

 less marked as we ascend in the succession. 



I believe that the facts obtained go to prove almost conclusively 

 that this is the case : and, moreover, that the majority of the rocks 

 now recognized as Pre-Cambrian were originally sedimentary strata, 

 which have undergone, during subsequent changes, a certain amount 

 of alteration, or metamorphism, but which still retain sufficient 

 evidence of their origin to warrant their being classified by the same 

 methods as those applicable to more recent deposits. That they can 

 in fact be distinguished by special characters due to the prevalence 

 of certain physical conditions at the time of deposition, by a well- 

 defined order in succession, and usually by a discordance in the 

 strike of the larger groups. 



In the physical history of our globe, nothing, perhaps, becomes 

 more evident than the well-recognized fact that its crust has been 



1 Read at the Meeting of the British Association at Sheffield, 1879. 



DECADE II. VOL. VI. — NO. X. 28 



