242 STRiTIGRAPHIC CATALOGUE. 



been interested in the work from the love of it, with no hope or 

 desire for pecuniary reward. Thus it happens that the investi- 

 gation at the outset is necessarily unequally developed. While 

 a state geological survey is able to collect a large amount of val- 

 uable material, fill in wide gaps and acqaire much information on 

 the subject, its direct efforts are for the most part incidental — 

 those observations which are made being in the course of other 

 investigations of an economic character. To this, as much as to 

 any other reason, perhaps, is due the great wealth of species 

 in the Lower Oirboniferous and Coal Measures, and the com- 

 paratively great paucity in some of the other formations. As 

 a further aid to unequal search for fossils is the unavoidable 

 difference of attention which the various formations receive, 

 especially in the early progress of the survey, owing to the 

 fact that all subjects cannot be commenced at once. Conse- 

 quently in carrying on the work on particular subjects, as for 

 example lead and zinc, or coal, certain geological formations 

 have more attention devoted to them in a single year, or even 

 during a few months, than others do in several years. The 

 fossils obtained from one horizon, even though they were 

 originally equally distributed, would surpass many times those 

 obtained during the same period from all the other formations 

 combined. 



In the unequal detection of fossils lies the second great 

 cause of disparity. Eocks present very different adaptations 

 to preservation. Sandstones usually are devoid of animal 

 remains, for the reason that percolating waters remove the 

 hard parts originally entombed. Many shales are practically 

 unfossiliferous. Some limestones contain no traces of life 

 whatever, while others are almost wholly composed of organic 

 remains. Coal, gypsum and certain other beds are also with- 

 out good fossils. On the other hand, beds which were origi- 

 nally very prolific with fossils often lose all or nearly all traces 

 through subsequent change in the composition of the rock- 

 Thus, highly fossiliferous limestones, in altering to dolomites, 

 have the organic remains largely obliterated in the process. 



