INTRODUCTION. 



may be stated, that such deposits unquestionably exist in North America, not only 

 as inferred from the publications of our countrymen Bigsby and Weaver, but also from 

 the works of several American authors who have described organic remains (Dekay, 

 Green, &c), as well as from the letters of Mr. Featherstonhaugh and Professor Rogers. 

 And although I have not yet seen a sufficient number of fossils to enable me to enter 

 into details, the identity of certain species of trilobites, common to that continent and 

 Great Britain, sufficiently sustains the accuracy of the inference. It is, indeed, probable 

 that the Silurian strata are distributed throughout that quarter of the globe, since Mr. 

 Charles Darwin has recently brought home from the Falkland Islands masses of rock, 

 charged with fossils which can with difficulty be distinguished from specimens of the 

 Caradoc sandstone, — while the existence of other Silurian Rocks in Southern Africa 

 has been established by the evidence of organic remains collected by Dr. Smith, and 

 first transmitted to me by Sir John Herschel. 



We have, therefore, every reason to believe, that although the complete order of the 

 foreign deposits of this age has not yet been pointed out, a little more labour will enable 

 us to place them in parallel with our own. 



But to return to England, — soon after I issued my first prospectus, Professor Sedg- 

 wick assigned to the older and contiguous rocks of Wales the name of Cambrian ; and 

 the reason is obvious ; for the strata so designated are not only conterminous with the 

 Silurian System, but are in several parts seen to rise from beneath its lowest beds, and 

 fairly to unite with them. These names are not, however, propounded as immutable ; 

 they are simply offered as the best means we possess of clearly defining the objects to 

 be attained. 



To show that these systems, like other large groups of strata in Great Britain, occu- 

 pied their true positions in the sea cliffs, both in relation to each other, and to the 

 younger deposits, I terminated my survey, by following them into Pembrokeshire, — 

 proving, that after a course of one hundred and sixty miles, they preserved the 

 same relation to each other and to the overlying deposits, as in Shropshire and Here- 

 fordshire. Professor Sedgwick will complete this portion of the task, by explaining 

 how the two systems range to the coast of North Wales, and how they are there 

 related to the younger formations. 



In tracing them to distant parts, the reader must not expect to find the Silurian 

 rocks preserving an uniform lithological character, or conforming precisely through- 

 out large spaces to the prevailing mineral types of the region described. This 

 remark applies not merely to other countries of Europe, but even to distant parts 

 of Britain : thus, although the calcareous flags of Llandeilo with their accompanying 

 schists are considered to form the base of the Silurian System, their place is some- 

 times taken, often indeed they are underlaid, by sandstones of considerable thickness. 

 Again, besides the frequent absence of beds of limestone in the Ludlow and Wen- 

 lock formations above alluded to, those deposits do not preserve the same lithological 



