PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



By Thomas Hexry Hlxley. 



[The foUowiug article was published, in 1865, iu " A Catalonjue of the Collection of 

 Fossilsin the Museum of Practical Geology," &c. Although evidently written, at least 

 iu part, long before its publication, it still remains one of the clearest and most com- 

 plete summaries of the subject yet published, and as the want of such a summary has 

 been frequently expressed, it is here reproduced. On account, however, of its length, 

 certain passages of simple local interest have been omitted. — H.] 



I. — Peeloiixary considerations. 



The formation of the collection of fossils in the Museum of Practical 

 Geology has been a necessary result of the operations of the geological 

 survey of Great Britain, whose ofdcers have been engaged for many 

 years past in determining the structure of the British islands ; that is, iu 

 ascertaining what is the nature and the order of superpOvsition of the 

 various irregular masses or regular " strata,''* piled one upon another, 

 which compose these like all other parts of the earth's crust. 



If rocks and stones were soft and easily cut, nothing would be easier 

 than the solution of these questions. It would be merely necessary to 

 make a sufficiently deep vertical cutting of the country in any required 

 direction, and the true order of the beds would be at once visible on 

 the walls of the section. But it is needless to say that iu practice cut- 

 ting into rocks is a very difficult and a very expensive operation, and 

 that the making of such artificial sections as these, for geological pur- 

 poses, is wholly out of the question. The geological surveyor is, there- 

 fore, obliged to trust very largely to the accidental occurrence of natural 

 sections, such as are afforded by the sea cliffs or the scarped hills which 

 may occur in his line of work, and to such artificial aids as are inciden- 

 tally yielded by the sinking of shafts or the cutting of railroads. 



It becomes, consequently, of essential importance to him to possess a 

 means of identifying the beds which he finds in one section with those 

 in another. Similarity or dissimilarity of miueralogical composition will 

 not always help him, as this quality not only varies in the same stratum, 

 but is similar in widely different strata ; so that beds of limestone in one 

 place may correspond as regards age and position with sandy or clayey 

 strata elsewhere. On the other hand, the continuity of a stratum be- 

 tween any two points examined would be clear aud decisive as to its 

 identity at the two points, but this evidence, for the reasons Just stated, 

 is but rarely attainable ; and where, as so frequently ha]ipens, the strata 

 have been disturbed from their original position, widely separated, 

 or partially destroyed between the two points, it becomes hopeless to 

 seek for any such proof. AVere there no other test of the nature of a 

 stratum at any given point than its mineral character, and its continuity 

 with some other stratum whose place iu the series was known, we might 



*Str.vtusi. — A single layer of the earth's crust, whatever its composition, is techni- 

 cally termed a stratum. For simplicity's sake, the often highly irregular nnisses of 

 igneous rock which enter largely into the composition of the earth's crust, and which 

 might not technically be termed ''strata," may be left out of consideration. 



