106 THE DAWN OF LIFE 



Still, as astronomers have suspected the existence of unknown 

 planets from observing perturbations not accounted for, and 

 as voyagers have suspected the approach to unknown regions 

 by the appearance of floating wood or stray land birds, antici- 

 pations of such discoveries have been entertained and ex- 

 pressed from time to time. Lyell, Dana, and Dr. Sterry Hunt 

 more especially have committed themselves to such specula- 

 tions. The reasons assigned may be stated thus : 



Assuming the Laurentian rocks to be altered sediments, 

 they must, from their great extent, have been deposited in the 

 ocean ; and if there had been no living creatures in the waters, 

 we have no reason to believe that they would have consisted of 

 anything more than such sandy and muddy debris as may be 

 washed away from wasting rocks originally of igneous origin. 

 But the Laurentian beds contain other materials than these. 

 No formations of any geological age include thicker or more 

 extensive limestones. One of the beds measured by the 

 officers of the Geological Survey is stated to be 1,500 feet in 

 thickness, another is 1,250 feet thick, and a third, 750 feet; 

 making an aggregate of 3,500 feet. 1 These beds may be traced, 

 with more or less interruption, for hundreds of miles. What- 

 ever the origin of such limestones, it is plain that they indicate 

 causes equal in extent, and comparable in power and duration, 

 with those which have produced the greatest limestones of the 

 later geological periods. Now, in later formations, limestone 

 is usually an organic rock, accumulated by the slow gathering 

 from the sea-water, or its plants, of calcareous matter, by 

 corals, foraminifera, or shell fish, and the deposition of their 

 skeletons, either entire or in fragments, in the sea bottom. 

 The most friable chalk and the most crystalline limestones 

 have alike been formed in this way. We know of no reason 

 why it should be different in the Laurentian period. When, 



1 Logan : " Geology of Canada," p. 45. 



