452 . FOOTPRINTS NEAR MONTREAL. [Ch. XXVII 



many fossil footprints have been observed on the surface of its rippled 

 layers. These impressions were first noticed by Mr. Abraham, of Mon- 

 treal, in 1847, and were supposed to be tracks of a tortoise; but 

 Mr. Logan has since brought some of the slabs to London, together 

 with numerous casts of other slabs, enabling Professor Owen to cor- 

 rect the idea first entertained, and to decide that they were not due 

 to a chelonian, nor, as he imagines, to any' vertebrate creature. The 

 Hunterian Professor inclines to the belief that they are the trails of 

 more than one species of articulate animal, probably allied to the King 

 Crab, or Limulus. Between the two rows of foot-tracks runs an im- 

 pressed median line or channel, supposed by the professor to have 

 been made by a caudal appendage rather than by a prominent part 

 of the trunk. Some individuals appear to have had three, and others 

 five pairs, of limbs used for locomotion. The width of the tracks be- 

 tween the outermost impressions varies from 3J to 5^ inches, which 

 would imply a creature of much larger dimensions than any organic body 

 yet obtained from strata of such antiquity. Their size alone is therefore 

 important, as warning us of the danger of drawing any inference, from 

 mere negative evidence, as to the extreme poverty of the fauna of the 

 earlier seas. 



Mr. Logan informs us,* that the Lower Silurian strata and the Potsdam 

 Sandstone in Canada rest unconformably on a still older series of aqueous 

 rocks, which, as he says, may be Cambrian (Lower Cambrian, or, perhaps, 

 still older ?), and which include conglomerates and beds of limestone. In 

 both of these, nodules of phosphate of lime are frequently observed. That 

 these contorted rocks are of aqueous origin, he infers from the presence of 

 quartz pebbles in the conglomerates. Together with the associated igne- 

 ous masses, this ancient series attains a thickness of at least 10,000 feet, 

 in the Lake Huron district, and includes the copper-bearing rocks of that 

 part of Canada. Below these again lies gneiss, with interstratified marble, 

 in which crystals of phosphate of lime both large and small are not un- 

 common. This phosphate, as Mr. Logan suggests, may have " a possible 

 connection with life in those ancient rocks." 



In the frontispiece to this volume, and in fig. 83, p. 59, the reader may 

 refer, to a section on the coast of Scotland where the Devonian strata lie 

 unconformably on the highly inclined Silurian schists, and I have cited 

 the eloquent reflections of Playfair when he looked, with his teacher 

 Hutton, " so far into the abyss of time." But in the lake district of N. 

 America, the Potsdam Sandstone, forming the upper or horizontal series, 

 is older than even the inclined strata of St. Abb's Head in Scotland. In 

 Canada again, we behold the monuments of still another period in the 

 remote distance, attesting, as Playfair exclaimed, " how much farther the 

 reason may go than the imagination can venture to follow." 



Valley of the Upper Mississippi. Mr. Dale Owen has recently pub- 

 lished a graphic sketch, in his survey of Wisconsin (1852), of the lowest 



* Quart. GeoL Journ. voL viil p. 210. 



