350 JDr. C. Callaioay — On the Archcean Rods. 



occurrence would in no wise affect my conclusion that the rock is 

 Lower Silurian." " Creatures of that low type of life may well 

 have lived on from the Laurentian to the Lower Silurian epoch." 

 Without stretching our faith quite so far, we may readily admit that 

 such lowly organisms have no very decisive stratigraphical value. 



Besides Eozoon, certain other traces of animal life have been 

 quoted from Archaean quartzites and limestones. They are generally 

 tracks or perforations such as may have been made by worms 

 crawling over the surface of sand or calcareous mud, or boring for 

 themselves vertical or curved burrows. Such evidences of life are 

 found abundantly in many subsequent formations, and they do not 

 materially differ from the tracks and burrows which abound on the 

 sea-shore at the present day. They are therefore of but slight value 

 in the correlation of rock-groups. 



We come next to the second test of contemporaneity, order of super- 

 position. This also can only be applied in some cases. Inversion is 

 not at all infrequent in these old rocks. The beds are sometimes 

 bent up into a series of folds, compressed so closely as to be almost 

 vertical, and the tops of the arches are thrown over in one direction, 

 so that the strata seem to dip regularly to the same point of the 

 compass. These phenomena are sometimes seen in Palaeozoic or 

 even Neozoic formations, but they are much more common in the 

 Archaean groups. In the United States large tracts of country are 

 occupied by gneiss with a prevailing south-east dip, but really made 

 up of numerous parallel folds with their summits thrown over to 

 the north-west. The contorted schists of Anglesey display the same 

 phenomena. In the region to the north-west of the Menai Straits, 

 these repetitions by folding falsely suggest a great thickness of 

 strata. The true relations of the beds are only to be ascertained by 

 disentangling the complications. The key to the solution of the 

 difficulty is the discovery of a grey gneiss underlying the prevailing 

 dark green schist of the district. As the strata roll over, the grey 

 band is here and there brought up to the surface, flanked on each 

 side by the green schist, which is seen to lie on the gneiss in sharp 

 synclinal folds, and the thickness, which is not great, can be thus 

 ascertained. 



Another difficulty in the application of the superposition test 

 is the excessive faulting which Arch^an rocks have suffered. In 

 some areas, tliese old formations had been heavily fractured even 

 before the Cambrian period, and they have of course been exposed 

 to all the dislocations which have subsequently rent the crust. Those 

 who are familiar with field-work only in the south or east of Eng- 

 land would feel strange on one of the Archaean battle-grounds, where 

 fire and water have contended which should most effectually remove 

 the traces of the original structure of the rocks ; but the tangled 

 maze of faults which have sliced and broken the crust would be 

 found the greatest stumbling-block to success. At St. Davids, 

 happily, the Lower Cambrian rests upon the upturned edges of the 

 Pebidian, and the Arch^an age of the latter is thus distinctly proved, 

 but such clear evidence of infraposition is rarely to be obtained in 



