THE OlMioVK IAN SYSTKM ll'.') 



In North Wales they are typically developed near liala, but 

 the base of the series has not yet been determined by graptoliti-- 

 evidence. There is a great thickness of sandy shales below the 

 Bala limestone, and some portion of them is probably of Bala age, 

 hut the greater part of the Lower Bala Group of Sedgwirk i.- now 

 regarded as Llandilian. 



The beds which have been grouped as Bala limestone include, 

 two distinct horizons or zones ; and the name Bala limestone is now 

 restricted to the lower of these zones, the Rhiwlas limestone, which 

 was formerly exposed at Rhiwlas, north of Bala, being tin- 

 equivalent of the Sholeshook limestone in Pembrokeshire. 



Even the main mass of the Bala limestone is often split up into 

 several beds by the intercalation of calcareous sandstone or shale, 



Ki^. 35. SKI -nox TiiKni'i.ii TMI: uu.v J.I.MESTONK SKU; I.KI.I.II.HIN, 



.SOUTH OK IIAI.A I.AKK (Hlltldy). 



8. si, ah--; with l.'iitn'n" .ifrii-f". 4. Sandy >hales, triloMtrs. 



7. Coiirse sand, many fossils. :<. Sandy limestone, brachiopods, 



<>. Crystalline limestone. -'. Volcanic ash. 



5. Coarse sand like Xo. 7. I. Shales with Orthi*. 



as at Gellygrin, to the south of Bala Lake, as described by Mr. T. 

 Ruddy. 11 Here there is a lower limestone, 12 feet thick and 

 containing many fossils, succeeded by sandy shales with many of 

 the characteristic Bala trilobites, and some feet higher a hard, 

 massive, crystalline limestone 18 or 20 feet thick, the whole group 

 of beds (5 to 8 in Fig. 35) being about 50 feet thick. 



The characteristic fossils of the Bala limestone are Trinuclev* 

 concentricus, Asaphns Poirisi, Phacops iifi/nil/itn.^ KoHUtlonotou 

 bisulcatus, Orthis vespertilio, <>. .-/</' rift-roides, and 0. biforata. Thoce 

 of the Rhiwlas lime-stone are Cyrtoceras sonax, Orthocerus rayans, 

 Trinucleiis selicornis, Ainpyj- tutu /(//(.-, Kitrrinnni<s sw' 

 Staurocephalus clavifrons, Phacops Jironijtiiorti, Lfjitu mi f< nn 

 Sphoeronites Litchi, Echinosplui-m nnrnnt-nim, Heinicogmites ric/"'''-' 

 and other cystid Echinotlerm>. 



Above this limestone there is some 1500 feet of sandy shale, 

 the upper beds of which contain graptolites of Silurian (i.e. 

 Llandovery) species. The exact horizon at which the divi.-ional 

 line should be drawn has not yet been ascertained, but the thick- 

 ness of Upper Bala Beds is probably not less than 1000 feet. 



