136 THE DAWN OF LIFE. 



of fragments of Eozoon which are scattered through 

 the Laurentian limestones. In these the fossil is 

 sometimes preserved in the ordinary manner, with its 

 cavities filled with serpentine, and the thicker parts of 

 the skeleton having their canals filled with this sub- 

 stance. In this case the chambers may have been 

 occupied with serpentine before it was broken up. At 

 St. Pierre there are distinct layers of this kind, from 

 half an inch to several inches in thickness, regularly 

 interstratified with the ordinary limestone. In other 

 layers no serpentine occurs, but the interstices of the 

 fragments are filled with crystalline dolomite or mag- 

 nesian limestone, which has also penetrated the canals; 

 and there are indications, though less manifest, that 

 some at least of the layers of pure limestone are com- 

 posed of fragmental Eozoon. In the Laurentian lime- 

 stone of Wentworth, belonging apparently to the same 

 band with that of St. Pierre, there are many small 

 rounded pieces of limestone, evidently the debris of 

 some older rock, broken up and rounded by attrition. 

 In some of these fragments the structure of Eozoon 

 may be plainly perceived. This shows that still older 

 limestones composed of Eozoon were at that time un- 

 dergoing waste, and carries our view of the existence 

 of this fossil back to the very beginning of the Lau- 

 rentian. 



With respect to organic fragments not showing the 

 structure of Eozoon, I have not as yet been able to 

 refer these to any definite origin. Some of them may 

 be simply thick portions of the shell of Eozoon with 



