OP THE OBERMITTWEIDA CONGLOMERATE. 23 



Then a traverse across the gneissic series of the adjoining district 

 showed a similar sequence to that with which we are familiar in 

 other areas of Archaean rocks. A selection from the Pyrenees 

 and from Wales, from rocks referred to the Archaean, can hardly 

 be distinguished from those of Saxony *. The common gneiss, the 

 mica-gneiss, the limestones are almost identical. So far as there is 

 any value in what I have called syntelism, it is clear that we have 

 a similar sequence in all these districts. If this be so, we may just 

 remark that these types of Archaean rock are not the highest, and 

 conglomerates are less likely to be found associated with them 

 than with such newer rocks as those I have named the Bangor 

 Series. 



In the next place I observe that the character of the conglo- 

 meratic series is quite different. We cannot conceive of those 

 gritty pebble-beds having originated in the same manner, or having 

 subsequently undergone the same vicissitudes as the muscovite- 

 schists and granitoid rocks seen on either side of them. We turn, 

 therefore, to other areas and ask whether we have any parallel case 

 where the overlying fragmental beds have a general resemblance to 

 the rocks of doubtful age and origin from which they, by their 

 position, might be supposed to be derived. We are fortunate in 

 Wales in being able to examine basement beds of newer series 

 (Cambrian) in which there has not been much deformationof the 

 constituents by mechanical action, while within twenty miles we 

 find the same beds violently cleaved and the included pebbles drawn 

 out almost beyond recognition. We find among the locally varying 

 beds of the Cambrian of North Wales a rock very similar in cha- 

 racter to the conglomerate of Obermittweida, and, in a closely 

 adjoining area, the equivalent rock squeezed so that the included 

 pebbles are crushed and flattened. 



Let the accidents which happened to the conglomerate of Llanberis 

 or of Borthwen, in Anglesey, affect such a bed as the conglomerate of 

 Bangor Station, and we should have a rock identical in all essential 

 points with that of Obermittweida. In both we have pebbles of 

 felsitic and granitoid rock, either crowded together or occurring 

 scattered through a grey matrix, which sometimes weathers into an 

 irregular and granular surface, as if from the development of the 

 rolled fragments in the felspathic paste, and sometimes is gritty 

 from the predominance of the siliceous constituents of the granitoid 

 rocks. As from the nature of the case we must often have what 

 may be called a granite-arkose, a mica-gneiss-arkose, or a schist- 

 arkose, so we may have many a portion of any newer series made 

 up so directly of the constituents of the underlying rocks as to 

 resemble them in superficial characters. But generally a wide 

 search reveals the derivative character of the newer beds. 



* Collections from the three districts mentioned were exhibited at the meet- 

 ing and are now in the Woodwardian Museum at Cambridge. 



