FOSSIL ANIMALS 



331 



assumed that all the lead came from uranium, and that none of the lead 

 has been removed, the time required for the transformation can be 

 computed. On this basis, one computation gave certain Permian strata 

 an age of about 220,000,000 years, late Cambrian 400,000,000 years, 

 and several pre-Cambrian formations ages ranging from 900,000,000 to 

 1,800,000,000 years. The age of the earth itself has been variously 

 estimated by the same method to be 3 to 6 billion years. 



Fig. 276. — Cambrian brachiopods (left) and trilobite. {From Hussey, " Historical 



Geology.") 



Change of Animal Life. — How animals have changed during the 

 millions of years since life began can be indicated here only in a general 

 way. Most of the Archeozoic rocks are igneous (once molten), hence 

 could not bear fossils. Unicellular organisms are supposed to have 

 existed then, but there is little evidence of them. In the Proterozoic, 

 which witnessed two periods of glaciation at a number of places (Utah, 

 Canada), there are limy deposits undoubtedly produced by unicellular 



Fig. 277. — Ordovician graptolite (left) and snail. {From Hussey, " Historical Geology.") 



plants. Sponges, spicules, and a jellyfish which have been reported are 

 still somewhat in doubt. 



Cambrian presents a great outburst of animal life of many different 

 kinds. Most characteristic and most abundant were the brachiopods 

 and trilobites (Fig. 276). Some shale in British Columbia contains 

 marvelously preserved jeUyfishes, sea cucumbers, siliceous (glassy) 

 sponges, annelid worms, and Crustacea. There were so many kinds of 

 Cambrian animals that the preceding era must have included many ; but 



