406 G. Schuchert — Paleozoic Crustal Instability 



for the process which results in the broken and frag- 

 mented materials of mountains, or oroclasmata, such as 

 testify to the former presence of mountains elsewhere 

 than in the places where these oroclastic deposits now lie. 



It may be urged that the thick accumulations of sedi- 

 ments are not reliable in indicating the exact time of de- 

 formation. In other words, it is thought that mountains 

 may last longer than a geologic period, or at least con- 

 siderably longer than the time of a geologic series. We 

 may grant the uncertainty, yet a close analysis of the 

 inherent evidence, both physical and organic, in the for- 

 mations composing the record, indicates that the time 

 values deduced from the oroclastic deposits are certainly 

 as reliable as, and usually far more so than, those gained 

 by the method of determining orogeny from the study of 

 angular unconformities. 



Our correspondence with Blackwelder also shows that 

 we wanted to establish the time of origin and the duration 

 of the orogenies, and whether they can be grouped into 

 minor and major deformations. Or, as the speaker has 

 elsewhere stated, into disturbances and revolutions. The 

 opinion appears to be widely prevalent among geologists, 

 however, that there are all grades of orogenies, and some 

 good geologists further hold that the deformations may 

 not all develop toward or at the close of the periods and 

 eras. In regard to these opinions, and first as to the revo- 

 lutions, the writer remains as firm as heretofore in the 

 belief that they are developed only toward the close of the 

 eras, and that they do delimit them, but apparently not at 

 the accepted boundaries of our text-books. It appears 

 that during the long-enduring revolutions the provincial 

 and regional orogenies follow one another in succession, 

 and that similar movements appear more or less syn- 

 chronously on either side of the Atlantic realm. Not only 

 this, but at these times orogeny is far more widespread 

 and more intense than at other times. In consequence of 

 the marked protuberances of the lithosphere, we see 

 usually the development of much cooled and even glacial 

 climates that bring on fundamental changes in the life of 

 these times. Hence the term "critical periods," condi- 

 tions of the environment that bring on critical times for 

 the plants and animals and quicken greatly their evolu- 

 tion into the biotic dawn of a new era. 



Now let us look into some of the revolutions, and first 



