THE LAUEENTIAN EOCKS. 17 



long looked in vain for their actual discovery. Still, 

 as astronomers have suspected tlie existence of un- 

 known 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, anticipations of such 

 discoveries have been entertained and expressed from 

 time to time. Lyell, Dana, and Sterry Hunt more es- 

 pecially, have committed themselves to such specula- 

 tions. The reasons assigned may be stated thus : — 



Assuming the Laurentian rocks to be altered sedi- 

 ments, they must, from their great extent, have been 

 deposited in the ocean; and if there had been ao 

 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 geo- 

 logical age include thicker or more extensive lime- 

 stones. One of the beds measured by the officers of 

 the Geological Survey, is stated to be 1500 feet in 

 thickness, another is 1250 feet thick, and a third 750 

 feet; making an aggregate of 3500 feet.* These 

 bods may be traced, with more or less interruption, 

 for hundreds of miles. Whatever 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 lime- 

 * Logan : Geology of Canada, p. 45. 



C 



