194 THE DAWN OF LIFE. 



have been prepared. I have examined these with care, and 

 am prepared to affirm that the chambers in these specimens 

 are filled with a dark-coloured limestone not more crystalline 

 than is usual in the Silurian rocks, and that the chamber- 

 walls are composed of carbonate of lime, with the canals filled 

 with the same material, except where the limestone filling the 

 chambers has penetrated into parts of the larger ones. I 

 should add that the stratigraphical researches of Mr. Vennor, 

 of the Canadian Survey, have rendered it probable that the 

 beds containing these fossils, though unconformably under- 

 lying the Lower Silurian, overlie the Lower Laurentian of the 

 locality, and are, therefore, probably Upper Laurentian, or 

 perhaps Huronian, so that the Tudor specimens may approach 

 in age to Giimbel's Eozoon Bavaricum.* 



Further, the authors of the paper have no right to object to 

 our regarding the laminated specimen as " typical " Eozoon. 

 If the question were as to typical ophite the case would be 

 different ; but the question actually is as to certain well-defined 

 forms which we regard as fossils, and allege to have organic 

 structure on the small scale, as well as lamination on the large 

 scale. We profess to account for the acervuline forms by the 

 irregular growth at the surface of the organisms, and by the 

 breaking of them into fragments confusedly intermingled in 

 great thicknesses of limestone, just as fragments of corals 

 occur in Palaeozoic limestones ; but we are under no obligation 

 to accept irregular or disintegrated specimens as typical ; and 

 when objectors reason from these fragments, we have a right 

 to point to the more perfect examples. It would be easy to 

 explain the loose cells of Tetradium which characterize the 

 bird's-eye limestone of the Lower Silurian of America, as 

 crystalline structures; but a comparison with the unbroken 

 masses of the same coral, shows their true nature. I have for 

 some time made the minute structure of Palasozoic limestones 



* I may now refer in addition to the canals filled with calcite and 

 dolomite, detected by Dr. Carpenter and myself in specimens from 

 Petite Nation, and mentioned in a previous chapter. See also 

 Plate VIII. 



