328 A. GEIKIE ON THE SUPPOSED 



rocks, that he had nothing to withdraw. He protested against the 

 rock in question being called granite. It had neither the minera- 

 logical composition nor the structure of granite. Many of the 

 supposed quartzite pebbles, on careful examination, proved not to 

 be such, but a rock similar to Dimetian granitoid rocks. 



Mr. Peach maintained that the Survey had never mapped the 

 beds at Nun's Chapel as syenite, and insisted that many of Dr. 

 Hicks's supposed facts would not bear examination. It was not 

 true that they recognized no faults. At Ogof-Llesugn the conglo- 

 merate was found welded with and kneaded into the granite. 



Prof. Hughes said he would not criticise the details of the 

 boundary-lines, but confine himself to the larger question. He 

 pointed out that in this, as in other similar areas, there was at the 

 base of the Cambrian a group of rocks which, for our present 

 purpose, might be conveniently divided into two series, an upper 

 volcanic series and a lower gneissic series. Where the gneissic 

 series consisted of alternations of granitoid rock, true gneiss, and 

 schists, however pierced by elvans and other intrusions, it was 

 possible to make out the strike ; but where, as at St. David's, the 

 granitoid rock was developed at the expense of the schistose portions, 

 it was impossible to say which of the divisional planes coincided 

 with original bedding, and often difficult to feel sure about the 

 simply metamorphosed and the intruded portions. 



If these were the result of extreme metamorphism of the lowest 

 portion of a volcanic series, it was natural that they should be 

 variable, and that the next overlying series should be of irregular 

 character and occurrence. Higher up in the group there were beds 

 of ordinary volcanic ejectamenta less altered. 



These types, in some form or other, were generally found in every 

 Archaean area in Britain. 



The Cambrian basement beds rested irregularly on various parts 

 of this Archaean group, and contained fragments of the older rocks, 

 as might be seen in the conglomerate south of Clegyr, not always 

 chiefly of the immediately underlying beds ; for that depended upon 

 the drift of the shingle. The granitoid rocks, being the lowest, were 

 less exposed, and so furnished less of the material ; and that was 

 readily broken up and decomposed, so as to form a kind of Arkose, 

 which resembles and sticks close to the granitoid rocks when it rests 

 on them. He thought that there was an Archaean group at St. 

 David's cut off by an enormous break from the Cambrian. 



Professor Bonnet stated that, putting aside minor points, there 

 were three main questions on which he would venture a few 

 remarks. Pirst, as regards the Arvonian group of Dr. Hicks, he 

 had for some time both entertained and expressed the opinion that 

 Dr. Hicks had been unfortunate in creating this formation, because 

 in it he had included rocks of very different characters, and pro- 

 bably very different ages. Secondly, as regards the " Dimetian " 

 of. St. David's, two points were raised by the author touching the 

 nature and the age of the rock. It was no doubt most difficult to 

 decide whether the rock was igneous or metamorphic. Some of 



