122 



STRATIGRAPHICAL GEOLOGY 



Archaean rocks. The lowest horizon from which fossils have been 

 obtained are sandstones overlying the basal conglomerate near 

 Llanfaelog (see Fig. 34). These fossils are an Orthis resembling 

 0. Carausi and fragments of a trilobite regarded by Professor 

 Hughes as a Neseuretus, but the specimen is probably Calymene 

 Tristani, while the slates which succeed the sandstones contain 

 graptolites of Upper Llanvirn species. Moreover, as Dr. Matley 

 has pointed out, there is a complete absence both in Central and 

 Northern Anglesey of the characteristic Arenig graptolites. 8 He 

 therefore infers that the lowest fossiliferous rocks in Central 

 Anglesey are equivalents of the beds now classed as Llanvirnian, 

 while in the north of the island even these do not appear, and 

 must have been overlapped by the Bala Series. 



Tan-y-bry 



Fig. 34. SECTION WEST OF LLANFAELOG (Dr. H. Hicks). 



Scale 6 inches to a mile. 



G. Blown sand. 



5. Black slates and dark flags. 



4. Flagstones and sandstones. 



3. Basal conglomerate (? Ordovician). 

 1. Granitoid Series. 



Llandilo Series. The typical area of these beds and the 

 place from which they take their name is Llandilo in Carmarthen- 

 shire, where the series originally described by Murchison as the 

 " Llandilo flags " forms a natural group, well marked off, both in 

 regard to ite lithological characters and its fossil contents, from the 

 underlying Llanvirn Series. Hence the Geological Survey have 

 recently found it necessary (as already stated) to adopt the latter 

 series as a primary division and to restrict the Llandilo Series 

 within its original limits. 9 



Near Llandilo and north-eastwards to Llangadock the group 

 consists mainly of grey calcareous flagstones weathering to a 

 yellowish brown. These are about 2500 feet thick, and in their 

 lower part they contain many beds of hard bluish -grey limestone, 

 most of them being fossiliferous and yielding both Ogygia Buchi 

 and Asaphus tyrannus. Under these beds is a hard massive 

 calcareous grit the Ffairfach grit, which has a thickness of about 

 150 feet and also contains Asaphiis tyrannus. 



