J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



exists to restrict, in thought, the crust movements to the end of a divi- 

 sion, be it epoch, period, or era; whereas in reality the movements pulse 

 through a division, the larger oscillations subsiding in the stage of quiet, 

 rising in increasing pulses to that culmination which marks the close of 

 one division of time, then subsiding through the initial portion of the 

 following division. 



The stages of crust movements, according to the nature of composite 

 rhythms, are grouped into diastrophic crescendoes and diminuendoes. 

 Thus in the great clastic formations of the lower Cambrian, laid down 

 chiefly in geosynclines, we see recorded the decadence of a great period of 

 revolution which terminated the Precambrian. The mudstones and sand- 

 stones of the upper Ordovician show a reawakening of internal forces 

 which led to the Taconic disturbance or revolution. The Silurian was a 

 period of quiet as compared to the Devonian, and the Mississippian was 

 quiet in comparison with the Pennsylvanian and Permian. The compara- 

 tive quiet of Triassic and Jurassic times led up in the close of the Jurassic 

 to a diastrophic movement of marked proportions in the Cordillera which 

 subdivides the Mesozoic era into an older and a younger division; but 

 this was followed by the still greater diastrophism of the Cenozoic, awak- 

 ening as the Laramide revolution, and rising to even greater magnitude 

 in the Neocene. 



Thus, there appears to run through geologic time a recurrence of greater 

 crescendoes which in their average period approach in round numbers to 

 200,000,000 years. This is seen in the Precambrian, in that the post- 

 Bottnian-pre-Kalevian era is found, apparently with accuracy, by means 

 of uranium minerals, to comprise 200,000,000 years. The following post- 

 Kalevian-pre-Paleozoic time, covering about 400,000,000 years, has im- 

 portant granite intrusions within it, and is marked in its earlier and later 

 halves by contrast, in the nature of sedimentation, which serve to sub- 

 divide it. The Paleozoic, whose length is not well known, but which may 

 comprise about 400,000,000 years, has been similarly subdivided, though 

 the place and number of the division lines have not been chosen alike by 

 all geologists. Dana used the end of the Ordovician as the place of sepa- 

 ration, calling the earlier section the Eopaleozoic, the later section the 

 Neopaleozoic. These sub-eras are broken into either two or three well 

 marked groups of periods. 



In so far as diastrophism serves as a basis for correlation, the Lara- 

 mide revolution includes the Paleocene, Eocene, and Oligocene epochs and 

 constitutes one period — the Paleogene or Eogene of Schuchert. It corre- 

 sponds to the Pennsylvanian in the sequence of the ISTeopaleozoic periods. 

 The Miocene, the beginning of the TvTeogene, witnessed the birth of the 



