Notices of Memoirs — Prof. W. C. Williamson. 461 



5. The granitoid rocks, felstones, and porpliyries, forming the 

 Eivals (yr Eifl) range of mountains. 



6. The so-called altered Cambrian rocks to the west of the Peny- 

 groes porphyry. 



7. The so-called intrusive granite in Anglesea, and the whole of 

 the area marked as altered Cambrian in that island. In addition to 

 these he has also extended some of the areas and defined more 

 clearly the order of superposition of these rocks in Pembrokeshire. 

 In North Wales, as in South Wales, he found that the pre-Cambrian 

 rocks resolved themselves into three well-marked and very distinct 

 types, and that these indicated sej)arate formations, each of which, 

 on careful examination, and when found in juxtaposition, proved to 

 be unconformable to the other. At St. David's the granitoid rocks 

 occur at the base, and, resting unconformably upon these, are found 

 the quartz-felsites. These are again succeeded unconformably by 

 the agglomerates, breccias, greenstone bands, and schists of the 

 Pebidian group. 



In North Wales this was also exactly the order in which the 

 various rocks were found to succeed each other, but the middle or 

 quartz-felsite group was found more largely developed in Caernar- 

 vonshire. 



As this middle group had not previously been separated under a 

 distinguishing name, the author now proposed to adopt for it the 

 name Arvonian, from the Eoman name Afvonia, and from which the 

 present name of Caernarvon is derived. So many of the large ridges 

 and lofty mountains of Caernarvonshire are composed of these felsitic 

 rocks, that it appeared to the author and his friends that this name 

 would be very appropriate for the formation. The distinguishing 

 characters most marked in these three pre-Cambrian formations may 

 be briefly summed ujd as follows : — 



1. Dimetian : Granitoid gneiss and quartzose rocks. 



2. Arvonian: Quartz, felsites, and porphyries (Helleflinta of 

 Torrel; petro-silex rocks, Hunt). 



3. Fehidian : Green and purple agglomerates and breccias, green 

 chloritic schists, with massive greenstone bands, talcose schists, etc. 



In these formations the bedding is usually easily recognized, but 

 at present the actual stratigraphical thickness cannot be correctly 

 estimated. It is perfectly clear, however, from the sections ex- 

 posed, that each must have a vertical thickness of many thousand 

 feet. That they have a very extended geological distribution over 

 the British Islands is also daily becoming more and more evident. 



lY. — On the Supposed Eadiolakianr and Diatomace^ of the 

 Coal-Measukes.^ By Professor W. C. Williamson, F.E.S. 



THE author called attention to the Traquarice of Mr. Carruthers, 

 found in the Lower Coal-measures of Lancashire and Yorkshire, 

 which small spherical objects that observer believed to be Eadio- 

 larians like those still living in existing seas. The author showed 



1 Read at a Meeting of the British Association, Dublin (before Section D, 

 Biology), August 19, 1878. 



