RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. '273 



It is true that fossil remains of certain species of land animals and 

 plants may be, and often are, found only within the limits of a certain 

 formation. In that respect they maybe regarded as among its char- 

 acteristic fossils, but the time range of a land fauna or flora is likely 

 to have fallen short of, or to have exeeeded that of an aquatic fauna 

 whose own duration is known to have been at least in part contempo- 

 raneous, because the physical conditions which were the principal fac- 

 tors in establishing and extinguishing an aquatic fauna would not nec- 

 essarily have materially affected the existence of adjacent and contem- 

 poraneous land faunas and floras. These questions, however, will be 

 more fully referred to in following essays. 



The foregoing remarks concerning the characterization of formations 

 have been made with special reference to those which are more or less 

 fossiliterous. It sometimes happens, however, that fossils do not exist, 

 or are not discovered, in certain formations which are evidently of sed- 

 imentary origin. This may have been due in some eases to the uncon- 

 geniality as a fauna! habitat of the waters in which the formation was 

 deposited, and in others to their failure to receive any fossilizable re- 

 mains of animals and plants from the laud. In other cases the absence 

 of fossils may have been due to their destruction or obliteration. The 

 latter has probably been the case with many metamorphic rocks and 

 with the great pre-Cambrian series of stratified rocks generally. In 

 all these cases the formations, while they may possess more or less dis- 

 tinct physical characteristics, lack the chief characteristics of sedi- 

 mentary formations, namely, the biological. 



The occurrence of an uufossiliferous sedimentary formation as a 

 member of an otherwise fossiliterous series is unusual, but in such a 

 case its definition and limitation would be effectually accomplished 

 by the underlying and overlying formations. In the case, however, 

 of a great uufossiliferous series of stratified rocks like the pre Cam 

 brian it is necessary to adopt a method for their study and classili- 

 cation based wholly upon physical data, after the fact that they 

 are pre-Cambrian has been determined from biological data. Such a 

 method of classifying and characterizing those uufossiliferous stratified 

 rocks as they occur in North America has been proposed by Prof. R. 

 D. Irving* and afterward elaborated by others. This great series of 

 rocks as it is developed on this continent has such distinguishing gen- 

 eral characteristics ami such magnitude and geographical extent that 

 some geologists have thought it worthy of being assigned to a special 

 division of study, but because no certain traces of organic forms have 

 been discovered in them they have, so far as it is now known, only the 

 indirect relation to biological geology that has just been referred to. 

 Still I regard it as not improbable that those strata were once fos- 



* Irving, R. D. : Classification of the Early Cambrian ;m<l pre-Cambrian forma- 

 tions. Seven tli Ann. Rep. IT. S. Geol. Survey, pp. 371-399, 



H. Mis. 114. pt. 2 IS 



