8o OUTLINES OF FIELD-GEOLOGY PART I 



either. The shaly bands, however, which lie above a 

 limestone are often more prolific than the limestone itself, 

 as the fossils can be extracted entire from the soft, sur- 

 rounding matrix. 



The inspection of a well-arranged series of fossils in a 

 museum, all cleaned and neatly labelled, affords but small 

 assistance in the practical work of finding the fossils in 

 the rocks. The learner must betake himself to the locali- 

 ties from which he knows that fossils have already been 

 obtained ; or if it is a district not yet explored for fossils, he 

 must carefully note first of all the characters of the rocks. 

 He will discover after some practice that it is not luck, 

 but skill and good eyesight, which make the successful 

 collector. Two observers may go over the same ground ; 

 one of them diligently applies his hammer, breaks up 

 innumerable blocks of limestone, finds not a single recog- 

 nisable trace of a fossil, and pronouncing the rock to be 

 unfossiliferous, passes on ; the other, perceiving the 

 calcareous nature of the stone, and therefore its possibly 

 fossiliferous character, puts his hammer in his belt, and 

 betakes himself at once to the weathered blocks. He 

 knows, as every one soon does who attends to the subject, 

 that in many cases a rock, which is really highly fossili- 

 ferous, may not appear to be so on a fresh fracture, where 

 the whole texture of the stone may be uniformly crystal- 

 line. But when exposed to the slow corrosive influence 

 of the weather, the difference between the molecular 

 arrangement of the calcareous matter in the organic re- 

 mains and of that in the surrounding matrix begins to 

 appear. Shells, corals, and crinoids stand out in relief 

 on the weathered stone, showing even some of their most 



