C. LAPWOBTH 02* THE MOFFAT SERIES. 330 



three thin rock-bands, each about a hundred feet in thickness, almost 

 inseparable mineralogically, and destitute of all traces of their former 

 inhabitants beyond a few Graptolites and Phyllopoda. 



There is, however, a complete answer to this objection. The 

 Lower Llandovery, of such vast extent in Cardigan and Merioneth, 

 has dwindled down to the thickness of the Birkhill Shales in the 

 intervening area of the Lake-district, where it forms the Coniston 

 Mudstones, a group of beds almost identical in thickness, lithology, 

 and palaeontology with the equivalent Scottish deposit of the Birk- 

 hill Shales. 



The Caradoc formation of Siluria, estimated by Murchison as 

 above 6000 feet in thickness in the typical area of Shropshire, is 

 reduced to less than half these dimensions in the Berywn Hills of 

 Merioneth. Here, also, it begins to take on the mineralogical cha- 

 racters of the Hartfell Shales, its lower beds, according to Prof. 

 Jukes, becoming more and more of the nature of black slate as we 

 approach the town of Conway*. In the Lake-district the whole 

 formation appears to be represented by less than 300 feet of calca- 

 reous shales. 



The same attenuation most certainly takes place also in the 

 underlying formations. The diversified Upper Llandeilo formation 

 (of Murchison), consisting in the typical areas in South "Wales of 

 several thousands of feet of schist, sandstones, and limestone, is 

 represented in the Berwyns and Arenigs by a homogeneous sheet of 

 dark shales of no great vertical extent. As we approach the shores 

 of the Irish Sea the formation has so thinned out as no longer to 

 be individually recognizable, the Lingula-flags, Tremadoc, Arenig, 

 Llandeilo, and Bala being all possibly included in the contracted 

 sections between the summit of Snowdon and the Lower Cambrian 

 beds to the east of Caernarvon. 



For is this extraordinary north-westerly attenuation of the Lower 

 Silurian rocks a phenomenon exclusively confined to Britain. On 

 the contrary, it is one of the most striking features of the Lower 

 Silurians of Europe in general. In Bohemia and Brittany, as well 

 as in Siluria, the Lower Silurian rocks are known to be of enormous 

 vertical dimensions, consisting everywhere of highly arenaceous 

 strata, rarely exhibiting any trace of true limestones. Further 

 to the northward, viz. in Bornholm and the Baltic provinces of 

 Russia, they have all dwindled to a thickness of less than a thousand 

 feet of impure limestones and schists. In Sweden and Norway, to 

 the extreme north-west, these limestones rapidly thin away, till 

 finally nothing remains but a few calcareous zones imbedded in a 

 sheet of dark Graptolite-schist f . 



* Jukes and Geikie's ' Manual of Geology,' 1872, p. 536. 



t We have an excellent illustration of the same fact in the Moffat district 

 itself. Each of the higher zones of the Moffat Series retains all its character- 

 istics absolutely unaltered when followed along the strike of the beds from N.B. 

 to S.W., but varies to a most remarkable extent in its successive reappearances 

 in the sections visible in the direction of the dip. The same rule holds good 

 even among the overlying greywackes to such a degree that several geographical 

 zones are recognizable, each with a distinct lithological character, and each 

 traceable in the line of strike from sea to sea. 



