508 A. Blytt on the probable Cause of 



UPPER TERTIARY. Cycle II. 



Oligocene. 



From 1,810,000 to 1,160,000 years before the present time. 



Arcs V-A'. Etage Ligurien, with 4 oscillations. 

 b'-l'. „ Tongrien, with 5 oscillations. 



Miocene. 

 From 1,160,000 to 700,000 years before the present time. 



Arc 8'. Etage Aquitanien ? 



9'. „ Langhien. 



10'. „ Helvetien. 



11'. „ Tortonien. 



12'. „ Messinien. 



Pliocene. 



From 700,000 to 350,000 years before the present time. 



Arc 13'. Etage Materin. 

 14'. „ Plaisancien. 

 15'. „ Astien. 

 16'. „ Arnusien. 



QUATERNARY. Cycle III. 

 From 350,000 years ago, to the present time. 

 Arcs l"-3". Etage Saharien, with 3 oscillations. 



The limits between the cycles of the curve are not drawn 

 arbitrarily. The beginning and the close of the first two 

 cycles are distinguished by their unusually low eccentricity. 

 The last arc in one cycle and the first in the following one 

 have, together, a duration of about 150,000 years; and in all 

 this time the eccentricity was very low. In these two cycles, 

 likewise, the highest mean eccentricity occurs in the middle 

 of the cycle. 



The Eocene period appears to have had 16 oscillations, and 

 should correspond to the first cycle ; the Oligocene, Miocene 

 and Pliocene have likewise together 16 oscillations, and cor- 

 respond to the second cycle. The Lower Eocene corresponds 

 to arcs 1-6, the Middle Eocene to 7-12, and the Upper 

 Eocene to 13-16. In the same way the Oligocene cor- 

 responds to arcs l'-7', the Miocene to 8'-12', and the Pliocene 

 to 13'-16'. There is thus a certain analogy between the older 

 and the younger Tertiary periods, ^e have here 6 divisions, 



