284 SCIENTIFIC APPARATUS. 



of the relative positions of the rocks, it taof die itmosl < on:.r<juence 

 to attend to the indications furnished by fossils. Suppose, i<r 

 instance, that among three great series of rocks, which may be 

 called A, B, and C, valuable coal seam:, i \i ,trd only in die upper 

 i::<-mbcr, C, though in general outward chau< tCl the whole three 

 series much resembled each other. It would probably be ioim<l 

 dial. the fossils in A differed from those in 15, and these again 

 from those in C. In following out the formations into other dis- 

 tricts, across intervening seas, lakes, morasses, or obscured -round, 

 or into tracts broken and confused by dislocations and the out- 

 pouring of much igneous rock, the geologist might easily become 

 <|nite at a loss to recognise any of the rocks by outward <:hara ui:,, 

 or to say whereabouts in the geological series his po iii<>n might 

 be; but if he succeeded in detecting recognisable fossils hi., dif- 

 ficulties would begin to diminish. If he : hould encounter a :<i 

 of rock;; with die fossils of A or B he could there entertain no 

 hope of coal, but would know that he was among the formations 

 lying below it. Further . search might reward him with traces of 

 the fossils of group C, and he would then feel that, whether or 

 not he should eventually discover the coal, he had, at 1< .1,1, found 

 the equivalents of the series of rocks in which that mineral occm nil 

 clsewl; 



Collections of typical fossils, that is, of those whirh are 

 specially characteristic of particular formations or subdivisions of 

 formations, have therefore an important place, both in the culti- 

 vation of original research in geology and in the teaching of the 

 science. When the eye is made familiar with these types it is 

 furnished with a clue to the disentanglement even of tin: dili'n ulL 

 .geological structure of an unexplored country. They supply 

 materials for the comparison of extinct forms of plant and animal 

 life with those still existing at die present lime; andulicn they 

 are drawn from different parts of the globe they afford the means 

 of tracing the outlines of the past history of living things upon our 

 planet. 



