February, 1919.| Annual Address. xvii 
have been determined for radio-active minerals occurring 
in a number of different geological systems and have given 
results ranging from 100,000 years in the case of minerals in 
rocks of the earth’s crust. The results thus given by radio- 
active minerals have recently been compared and combined by 
J. Barrell with those obtained from other geological methods 
of determining the age of the fossiliferous rocks, and the 
results are shown in the annexed table. 
MILLIONS OF YEARS. 
Minimum. | Maximum 
| ————————$__—__-—— 
cent 
Pleistocene } 1 | 1} 
Pliocene 6 nm 
Miocene 12 “4 
Oligocene 16 16 
ocene 20 96 
— 55 | a” 5665 
KAINOzoIc. | 
Cretaceous 65 85 
Jurassic 35 45 
ic 35 45 
135 —— 175 
MeEsozoic. 
Permian 
Carboniferous } ne ~ - 110 130 
Devonian i aa on 50 50 
ilurian if 40 
Ordovician as sa 90 130 
Cambrian es Et laps 70 110 
—— 300 nec 
PALZOZOIC. 
ToTaL fae 550 790 
ee a a ecli 
From this we may deduce that the trilobites ce ckasd feu 
between 550 and 700 million years ago, the first fish between 
350 and d 
raillion aan ago. The first unmistakable mammals ap- 
ared at about the same time as the birds, or possibly 
development in the Tertiary epoch and especially in_the 
Miocene and Pliocene periods, say, between five and ten million 
