426 Dr. C. Callaway — Hoio to Work the Archcean Hocks. 



whichi would otherwise remain uncertain. The oft-repeated objec- 

 tion that " we cannot study mountains in the microscope " is met by 

 the practical refutation that we have found out by its aid things 

 which the hammer, the lens, and the acid-bottle had not dis- 

 covered. The enfoi'cement of this point belongs, however, to 

 the professed microlithologist. 



The strike of a series has frequently been used as an aid in correla- 

 tion. It was long since pointed out by Murchison that the Lewisian 

 of Scotland has a general strike to the north-west, while the newer 

 gneiss series strikes to the north-east. It was rightly argued that 

 such a divergence of strike indicated a great difference in age. The 

 gneissic rocks of Shropshire and the Malvern Hills, resembling the 

 Lewisian as they do in their lithological structure, possess the same 

 strike, and the fact is held to confirm the correlation of the English 

 groups with the northern type. The Pebidians of St. Davids strike 

 about east and west, and this is the prevailing lie of the contem- 

 poraneous series in Anglesey. The volcanic rocks of Shropshire 

 also strike in the same direction. On the other hand, the volcanic 

 series of Charnwood Forest, presumed to be of the same age, has the 

 strike of the Lewisian. Other facts also tend to show that this test 

 is of uncertain value. The utmost we can safely assert is that if 

 there are other good reasons for believing two groups to be contem- 

 poraneous, a coincidence of strike will, within certain limits, add to 

 the weight of evidence. Strikes are not so uniform as has sometimes 

 been represented. Over large portions of the Northern Highlands 

 the strike of the Lewisian is not to the north-west, and that of the 

 newer gneiss series is not north-east. Also in Anglesey and Shrop- 

 shire the volcanic series frequently deviates widely from the normal 

 strike. These frequent divergences are probably due to faulting 

 and to the influence of intrusive masses. In large areas, in which 

 the original axes of upheaval have not been so broken and distorted 

 as to confuse their prevalent direction, the strike test will be of much 

 greater service than in a small isolated district, in which there is 

 not room for the principal lines of movement to assert their supremacy 

 over subsequent disturbances. 



In Archsean work the evidence is generally cumulative. Any one 

 test taken alone may not produce conviction. Mineral characters, 

 included fragments, strike, origin of deposits, state of alteration, 

 even similarity of succession, may not always remove a suspicion 

 that the resemblance is due to coincidence, or to a general cause 

 which may occur in more than one group. But we are rarely left 

 to one line of proof, and every additional test strengthens the 

 evidence in an increasing ratio. Only field-work can supply the 

 full testimony. Each tap of the hammer may help to build ujd the 

 conclusion. The results grow as section after section is observed, 

 and hundreds of minute facts, which cannot be published, or even 

 recorded without incalculable labour, gradually bring home con- 

 viction to the mind. 



In concluding these observations, the writer would strongly insist 

 upon the necessity of thorough and detailed work. Where the 



