366 CAMBRIAN PERIOD 



to recognize (i) rather abrupt changes brought about by over- 

 whelming invasions; (2) less abrupt changes brought about by 

 the more gradual ingress of outside species, and the gradual com- 

 mingling of immigrants with resident species; (3) very gradual 

 changes due to the slow evolution of resident species when not much 

 affected by immigration or by physical changes; and (4) rapid 

 evolution due to profound changes in the physical conditions or to 

 other agencies less well understood. 



The abrupt appearance of the Cambrian fauna. The apparent 

 suddenness of the appearance of the Cambrian fauna is unexplained. 

 In a general way, it may be said that older formations have been 

 metamorphosed, and that this destroyed most of their fossils; but 

 this suggestion is not altogether adequate, for some of the older 

 formations are not greatly changed, and some younger metamorphic 

 rocks carry fossils. It is also true that some younger formations 

 which seem well suited to receiving and retaining organic impres- 

 sions are without them. Geologists are inclined to refer the scanti- 

 ness of pre-Cambrian fossils, and hence the apparent abruptness of 

 the introduction of the Cambrian fauna, to unfavorable conditions 

 for fossilization in pre-Cambrian time, combined with subsequent 

 changes in the rock. This makes the abruptness a matter of rec- 

 ord, rather than of fact. 



Map work. Suggestions for work with geologic folios are found in Laboratory 

 Exercises in Structural and Historical Geology, SALISBURY AND TROWBRIDGE, 

 Exercise VIII. 



