Introduction to Geology, 73 



" Solid beds," observe Messrs. Cuvier and Brongniart, " are 

 no longer formed, in any magnitude, in our present seas, and 

 the species of shells are always the same in the same parts of 

 the sea. For example, since they fished for oysters upon the 

 coast of Cancale, for pearls in the Gulf of Persia, &c., it is 

 not seen that these sliells have disappeared, to be replaced by 

 other species. From researches, made by one of us, it appears 

 that, during near 2000 years, the bottom of those seas has 

 not been changed ; that it has not been covered with any new 

 bed ; and that the species of shells, which were fished then, 

 live and are fished even at the present day." 



It has been stated, that one of the most useful discoveries 

 resulting from the investigation of the secondary strata is that 

 universally prevailing rule, that similar strata contain similar 

 extraneous fossils. However curious these remains might be 

 before considered, as memorials of extinct tribes and of mighty 

 changes, they became more valuable to the geologist when he 

 had ascertained that certain organic remains were peculiar to 

 certain formations, and that these bodies furnished him with 

 data by which to trace and to determine the identity of such 

 formations in remote parts of the world. Experience has fully 

 proved that these characteristics are infinitely more to be re- 

 lied on than any with which we are acquainted. Chemical 

 or mineralogical distinctions are far from universal : they are 

 often modified, and sometimes wholly changed, even in limited 

 areas ; their practical application for geological tests has been 

 repeatedly found delusive. Not so the extraneous fossils : the 

 same genera of vegetables and Testacea, for instance, which 

 characterise the carboniferous limestones and shales of the 

 English coal fields, may be observed in similar formations at 

 Spitzbergen, in numerous parts of the European continent, in 

 North America and Van Dieman's Land. The Trilobite of 

 our transition limestone occurs in France, in North America, 

 and other remote localities ; and the fossils of the transition 

 slate of Wales are repeated in Germany, in France, Colom- 

 bia, New York, Pennsylvania, and near Lake Oneida. 



All the observations of scientific travellers tend to confirm 

 this important law. By its aid we are enabled to construct 

 geological maps with perfect precision. Those which have 

 been compiled by Messrs. Smith, Greenough, Farey, Buck- 

 land, Webster, Weaver, and other geologists of well-known 

 reputation, owe their accuracy mainly to the observance of this 

 principle. Geological surveys have now been made nearly 

 over the whole of our island. District' surveys have frequently 

 delineated the superficial boundaries of the formations, with as 

 much precision as the direction of our roads and rivers. Some 



