

172 J. D. Dana — Sedgwick and Murchison. 



pages, with twenty-seven plates of fossils, and nine folded 

 plates of stratigraphical sections, besides many plates in the 

 text — the outcome of his eight years of work. Five hundred 

 pages are devoted to the Silurian System. 

 The dedication is as follows : 



"To you, my dear Sedgwick, a large portion of whose life has 

 been devoted to the arduous study of the older British rocks, I 

 dedicate this work. 



Having explored with you many a tract, both at home and 

 abroad, I beg you to accept this offering as a memorial of friend- 

 ship, and of the high sense I entertain of the value of your 

 labours." 



Through Murchison's investigations here recorded, as he 

 remarks in his Introduction with reasonable satisfaction, "a 

 complete succession of fossiliferous strata is interpolated be- 

 tween the Old Red Sandstone and the oldest slaty rocks." He 

 observes as follows of Sedgwick: "In speaking of the labours 

 of my friend, I may truly say, that he not only shed an entire- 

 ly new light on the crystalline arrangement or slaty cleavage 

 of the North Welsh Mountains, but also overcame what to 

 most men would have proved insurmountable difficulties in 

 determining the order and relations of these very ancient strata 

 amid scenes of vast dislocation. He further made several 

 traverses across the region in which I was employed ; and, 

 sanctioning the arrangement I had adopted, he not only gave 

 me confidence in its accuracy, but enhanced the value of my 

 w T ork by enabling me to unite it with his own ; and thus have 

 our joint exertions led to a general view of the sequence of the 

 older fossiliferous deposits." In accordance with these state- 

 ments many of the descriptions and the very numerous sec- 

 tions represent the Cambrian rocks lying beneath the Silurian, 

 — though necessarily with incorrect details, since neither Mur- 

 chison nor Sedgwick had then any appreciation of the ac- 

 tual connection between the so-called Cambrian and Silurian. 



The Silurian system, as here set forth, is essentially that of 

 Murchison's earlier paper of 1835 ; and through the work, as each 

 region is taken op, the rocks of the Upper and Lower divisions, 

 and their several subdivisions, are described in order, with a 

 mention of the characteristic fossils As to the relations of 

 the two grand divisions, he says that " although two or three 

 species of shells of the Upper Silurian rocks may be detected 

 in the Lower Silurian, the mass of organic remains in each 

 group is very distinct^ Later he makes the number of iden- 

 tical species larger ; but even the newest results do not in- 

 crease it so far as to set aside Murchison's general statement 

 of 1838. 



