§ 11. SUBDIVISIONS OF THE SILURIAN SYSTEM. 



767 



The whole of these strata are of marine origin, without 

 any interspersions of fresh-water or terrestrial detritus 

 whatever ; and the limestones swarm with trilobites, cri- 

 noidea, corals, spirifers, leptrenae, and other fossils, with 

 which our late investigations have made us familiar. The 

 subdivisions introduced are locally important ; but a general 

 analogy prevails in the organic remains throughout the 

 entire system, and there does not appear to be any essen- 

 tial variation in the forms or conditions of organic life, as 

 deducible from the fossils, from the commencement to the 

 termination of the series ; and though each principal divi- 

 sion may be distinguished by its peculiar fossils, yet the 

 upper and lower Silurian rocks are bound together by 

 species common to both, and form but one natural system, 

 though for convenience they are classified in two groups. 



11. Subdivisions of the Silurian System. — The 

 Upper Silurian comprises the Ludlow rocks, so named from 

 their great development around Ludlow, in Shropshire ; 

 they are subdivided as follow : — 



(a.) Upper Ludlow, consisting of micaceous and calca- 

 reous sandstone, decomposing into soft mud : a few species 

 of shells common in the lower Devonian occur, and 

 some (Orthis orbicularis) that have not been observed in 



&c. In two parts, royal 4to. with a separate map, and numerous illustra- 

 tions. London, 1839 ; p. 768. The publication of this splendid work 

 formed an era in British Geology : it is a noble monument of patient, 

 laborious, and successful scientific research, pursued through a long- 

 series of years, regardless of toil, time, or expense. The results of the 

 labours of its highly-gifted author are alike novel and important : rocks, 

 which, under the names of transition and greywacke (terms that served 

 as a veil for our ignorance), were previously considered without the 

 pale of scientific arrangement, were for the first time reduced to a 

 regular system, and their zoological characters as well defined as those 

 which mark the newer secondary formations. The addition made to 

 the geological fauna by the author, amounts to nearly 400 species. 

 This is truly a national work : the description of the British coal-fields 

 is as important in an economical, as in a scientific point of view. 



