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 alw ays the same in the same parts of 
the sea. Tor 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 shells have disappeared, to be replaced by 
other species. From researches, made by one of us, it appears 
that, during near 2000 years, ‘4hie 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 
univer sally prevailing rule, that similar strata contain similar 
extraneous fossils. Howe ever 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 idertity 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 miner alogical distinctions are far from universal: they are 
often modified, and sometimes wholly changed, even in limited 
areas; their practical application for seolog ical tests has been 
repeatedly found delusive. Not so the extraneous fossils : the 
same genera of vegetables and Testacea, for instance, which 
Ghevactense the carboniferous limestones and shales of the 
English coal fields, may be observed in similar formations at 
Spitzber gen, 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 eliservations 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 ceologists 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 ourisland. District surveys have frequently 
delineated the superficial boundaries of the fermations, with as 
much precision as the direction of our roads and rivers. Some 
