No. 2.] DAWSON — IMPRESSIONS AND FOOTPRINTS. 71 



rocks which have been referred to trails of Gasteropods, as for 

 instance, those from the Clinton group, described by Hall. 



An might be expected, the markings above referred to, when 

 in relief, occur on the under sides of the beds. A few instances 

 may, however, be found where they exist on the upper surfaces. 

 On careful consideration of these raised impressions, I have 

 arrived at the conclusion that they have been left by denudation 

 of the surrounding material, just as footprints on dry snow 

 sometimes remain in relief after the surrounding loose snow has 

 been drifted away by the wind ; the portion consolidated by 

 pressure being better able to resist the denuding agency. Such 

 markings in relief on the upper surfaces of beds are, however, I 

 believe, altogether exceptional. 



It seems idle to give specific names to markings of this kind. 



They have evidently been made by many different species of 



animals, but they afford no certain characters. Fig. 4 a to/, 



represents some of the forms most common in the Carboniferous 



beds. 



Imitative markings. 



Rill-marks are often very beautifully developed on the Car- 

 boniferous shales and argillaceous sandstones, though not more 

 elaborately than on the modern mud-banks of the Bay of 

 Fundy,* and they occur as far back as the oldest Cambrian. f 

 Some of these simulate leaves of ferns and fronds of Laminariae, 

 and others resemble roots, fucoids allied to Buthotrephis, or the 

 radiating worm-burrows already referred to. 



Shrinkage cracks are also abundant in some of the Carbonifer- 

 ous beds and are sometimes accompanied with impressions of 

 rain-drops. When finely reticulated they might be mistaken for 

 the venation of leaves, and when complicated with little rill- 

 marks tributary to their sides, they precisely resemble the Dic- 

 tuolites of Hall from the Medina Sandstone. 



An entirely different kind of shrinkage- crack is that 

 which occurs in certain carbonized and flattened plants, and 

 which sometimes communicates to them a marvellous resemb- 

 lance to the netted under-surface of an exogenous leaf (fig. 5). 

 Flattened stems of plants and layers of cortical matter, when 

 carbonized, shrink in such a manner as to produce minute 



* Acadian Geology, 2nd edition, p. 26. 

 •f Salter, Journal of Geol. Society, vol. 12, p. 251. 



