./. J>. Dana- -Sedgwick and Miik'hison. L69 



Ludlow rocks; (2) the Wenlock Limestone; (8) the Lower Lud- 

 low rocks; (4) Shelly sandstones, "which in Shropshire occupy 

 separate ridges on the southeastern flanks of the Wrekin and 

 the Gaer Caradoc"; (5) the Black Trilobite flagstone wh< 

 "prevailing Trilobite is the large Asajphus Buchii, which 

 with the associated species," he observed, "is never seen in any 

 of the overlying groups;" and, below these, (6), Red Con- 

 glomerate sandstone and slaty schist several thousand feet in 

 thickness. 



By the following January, L834, Murchison was ready with 

 a further report,* in which he described the " four fossilifer- 

 OUS formations" in detail, and displayed, on a folded table 

 arranged in columns, their stratigraphical order, thickness, sub- 

 divisions, localities, and " characteristic organic remains." 

 The subdivisions of the rock-series in the memoir are as fol- 

 lows, commencing above: I, Ludlow rocks, 2,000 feet; II, 

 TVenlock and Dudley rocks, 1,800 feet; III, Horderley and 

 May Hill rocks (afterward named Caradoc), 2,500 feet ; IV, 

 Builth and Llandeilo flags, characterized by Asaphus Buchii, 

 l,2oo feet; and, below these, V, the Longmynd and Gwas- 

 taden rocks, many thousand feet thick, set down as unfossil- 

 iferous. 



Thus far had Murchison advanced in the development of 

 the Silurian system by the end of his third year. Upper and 

 Lower Silurian strata were comprised in it, but these sub- 

 divisions were not yet announced. 



During the interval from 1831 to 1834, Sedgwick presented 

 to the British Association in 1832 a verbal communication on 

 the geology of Caernarvonshire, and another brief report of 

 progress in 1833. A few lines for each are all that was pub- 

 lished. The difficulties of the region were a reason for slow 

 and cautious work. 



In 1834, as first stated in the Journal of the Geological 

 Society for the year 1852, the two geologists took an excursion 

 together over their respective fields. Sedgwick says :f "I 

 then studied for the first time the Silurian types under the 

 guidance of my fellow-laborer and friend ; and I was so struck 

 by the clearness of the natural sections and the perfection of 

 his workmanship that I received, I might say, with implicit 

 faith everything which he then taught me." And further, 

 ** the wdiole 'Silurian system' was by its author placed above 

 the great undulating slate-rocks of South Wales. " The geol- 

 ogists next went together over Sedgwick's region, and the sec- 



* Murchison, Proc. Geol. Soc. ii, 13, 1834. The subject was also before the 

 British Association: Report for 1834, p. 652. 



f Sedgwick, Quarterly Journal of the Geological Society, viii, 152, 1852. 



