Dr. C. Callaway — Koiv to Work the Archcean Rocks. 421 



This test decreases in value as the formations compared in- 

 crease in distance. Purple rocks, like the Wrekin lavas, are found 

 about twelve miles to the west, in Pontsford Hill, and they may 

 reasonably be referred to the same series. But the same high 

 probability cannot be claimed for a certain purplish grit near 

 Bangor, which in hand specimens can hardl3'- be distinguished from 

 some of the Wrekin tuifs. It is much more likely that the same 

 conditions prevailed at the same time over an area of a few miles 

 round the Wrekin than that they did so from the Wrekin to the 

 Menai Straits. The very same volcanos might have poured forth 

 both the Wrekin and the Pontsford Hill lavas, but it is less probable 

 that volcanos were contemporaneously ejecting similar material in 

 both the Wrekin and Bangor areas. Were the two formations com- 

 pared separated by the breadth of the Atlantic Ocean, the probability 

 would be still further weakened. 



Several considerations must be taken into account in estimating 

 the value of this test. It is first of all necessary to ascertain that 

 the rock under investigation is older than the Cambrian. If this 

 cannot be proved, the evidence is much weakened. The rocks of 

 Charnwood Forest, for example, are overlain on all sides by strata 

 none of which are older than the Carboniferous, so that all we can 

 learn from superposition is that the Charnwood series is Pre- 

 Carboniferous. Lithologically, the formation has affinities both with 

 the Pebidian and with the Grreen Slates and Porphyries of the Lake 

 District, supposed to be Ordovician. If the Charnwood group could 

 be shown to be older than the Cambrian, the latter hypothesis would 

 obviously be excluded, and the Pebidian age of the series would 

 become almost certain. Even as it is, the mineral resemblances 

 between these rocks and the Pebidian are so great that there is high 

 probability of their contemporaneity. In Anglesey there is a great 

 series ^ of slates, shales, breccias, and conglomerates, which much 

 more closely resembles the St, Davids Pebidian than does the 

 Charnwood groups. They are older than the Cambrian, for the 

 Lower Cambrian contains pebbles derived from them. This narrows 

 the evidence considerably. We are acquainted with only two or 

 three Archgean groups in Britain, and as the Anglesey series is 

 Archgean, and has no resemblance to any other group than the Pe- 

 bidian, with which moreover its affinities are numerous and striking, 

 the proof of its Pebidian age is almost complete. Lithological 

 resemblance alone is not sufficient ; Pre-Cambrian age alone is not 

 sufficient, since there are several Pre-Cambrian groups ; but the 

 two combined form a fairly conclusive mass of evidence. 



Another important accessory test is similarity of succession. The 

 mineral resemblance between two groups of strata at some distance 

 from each other, though often reasonably satisfactory proof of con- 

 temporaneity, is sometimes open to uncertainty, owing to the possi- 

 bility of the resemblance being mere coincidence. The dark-green 

 schist on the Menai Straits, in Anglesey, is, for example, undis- 

 tinguishable from the dark schist (Fig. 1, e) of the central band. 

 ^ Described in Quart. Journ. Geol. Soc, May, 1881. 



