68 The Badland Formations of the Black Hills (Region 



to favorable places, where there has been considerable opportun- 

 ity for recent concentration. A sample from the northwest 

 corner of section 18, township 2 south, range 15 east, on North 

 Sag-e creek recently assayed at the School of Mines, showed a 

 value of forty cents a ton, equivalent to approximately forty- 

 nine cents a cubic yard. This in itself might seem to indicate 

 opportunity for fair return) for labor, but the concentrated 

 gravels are not plentiful and through much of the year favorable 

 water supply is lacking. 



FOSSILS 



Fossils as generally understood are the parts of animals and 

 plants living- before the present era that have been buried in the 

 rocks and preserved by natural causes. The manner and degree 

 of preservation vary greatly. The essential thing is the sealing 

 up of the remains in the rocks so that destruction and decay may 

 be prevented. Animals such as the ice-entombed mammals of 

 Siberia, or the amber enclosed insects of /the Baltic, are prac- 

 tically perfect as the day they were buried, but they are excep- 

 tional. Generally only the hard parts, such as bones or teeth, 

 or shells remain. Not infrequently these are replaced, particle 

 by particle by new mineral matter of some kind, particularly 

 silica or pyrite, then they become petrifactions. Sometimes only 

 the form or .the impression of the original parts are preserved, 

 hence the terms molds and casts. Occasionally the relics are 

 limited to footprints, or trails, or burrows, or borings, or eggs. 



Animals living in the water or frequenting marshy places 

 ?or food and drink are more easily and more quickly buried 

 beneath sediments, hence their fossils are usually more abund- 

 ant. The bodies of dry land animals are subjected to the vicis- 

 situdes of sun and rain and wind, and frost, and are often 

 feasted upon by scavenger birds and beasts and insects. Further- 

 more their burial is commonly brought about only during flood 

 season. All of these tend to the destruction or dismemberment 

 of the various parts. Again, even if once nicely buried, the} 

 may later be obliterated by metamorphism or be destroyed by 

 disintegrating and denuding agencies. As a result of all this, 

 the history of certain groups of animals is meagre in the ex- 

 treme and doubtless hordes of species have left no worthy evi- 

 dence of their ever having lived. 



EXTINCTION, EVOLUTION, MIGRATION 

 In the study of the life history of the fossil organisms 

 puzzling questions are continually arising for urgent answer. 



