74 THE PENOKEE IROXBEAKING SERIES. 



gists at reaching an nndevstanding of even their mineral composition. 

 Most of those wlio liad written upon them up to this time liad l)een unac- 

 quainted witli the newer petrographic methods, which indeed are only 

 recently reaching any very satisfactory development so far as the study of 

 these difficult rocks is concerned. Then again, the structural pi'oblems pre- 

 sented by these rocks are always among the most difficult the geologist has 

 to deal with. Exposed as they have been to the disturbing forces of all 

 the enormous lapse of ages since their first production, their occurrence in 

 anything like their normal position is the rare exception. Faulted, folded, 

 squeezed, internally altered in every possible way, and intruded in every 

 fashion Ijy everj' sort of eruptive material, the difficulties in the wa}' of a 

 correct understanding of even a small area of these ancient rocks, are often 

 well nigh insurmountable. Certainly they are to be overcome only Ijy 

 the most n^inutely accurate structural work. Approaching them from 

 different points of view, the geologist who has acquired his experience 

 among the unaltered sediments and he whose ideas have been developed 

 mainly among the more modern erujjtives, unrestrained by the accurate 

 knowledge obtainable only through the modern petrographic methods, and 

 through exact structural investigation, have arrived at the most opposite 

 conclusions with regard to the structural relations aiid genesis of these 

 ancient rocks. 



A careful sifting of all that has been written upon this subject was 

 certainly very desirable — a sifting- which should not attemt)t to destroy all 

 information gathered, l)ut which sliould try, so far as possible, to separate 

 what is inference only from what was a certain result of accurate observa- 

 tion. In the case of the present work, however, it becomes very evident 

 within the first few jiages that the authors have a distinct theory to ad^o- 

 cate with regard to the pre-Cambrian formations, a theory, in fact, for 

 many years past advocated b}' the older of the two authors. This theory 

 is simply that the pre-Cambrian rocks constitute a truly Azoic, confused 

 intermingling of sedimentaries and eruptives, which, thougli covering 

 in their production an immense lapse of time, are not divisible on any 

 correct geological principle into chronologically distinct terranes. All 

 evidence found tliat had liccu presented in opposition to this view is criti- 



