LAURENTIAN AND EARLY PALiEOZOIO. 



33 



out of mud and sand-banks left by the tide, and which 

 sometimes cover groat surfaces with the most elaborate 

 tracery, on the modern tidal shores as well as in some of 

 the most ancient rocks. Dendrophycus* oi Lesquereux 

 seems to be an example of rill-mark, as well as Aristophy- 

 cus, Clwphycus, and Zygophycus, of Miller and Dyer, 

 from the Lower Silurian. 



Rill-marks occur in very old rocks, f but are perhaps 

 most beautifully preserved in the Carboniferous shales 

 and argillaceous sandstones, and 

 even more elaborately on the mod- 

 ern mud - banks of the Bay of 

 Fundy.J Some of these simulate 

 ferns and fronds of Laminaria3, 

 and others resemble roots, fucoids 

 allied to Buthotrephis, or the ra- 

 diating worm-burrows already re- 

 ferred to (Fig. 10). 



Shrinkage-cracks are also abun- 

 dant in some of the Carboniferous 

 beds, and are sometimes accom- 

 panied 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 pre- 

 cisely resemble the Dictyolites of 

 Hall from the Medina sandstone 

 (Fig. 11). 



An entirely different kind of shrinkage-crack is that 

 which occurs in certain carbonised and flattened plants. 



Fig. 10. — Carboniferous rill- 

 mark (Nova Scotia), re- 

 duced, to illubtrute pre- 

 tended Alg8B. 



* " Coal Flora of Pennsylvania," vol. iii,, Plate 88, 

 \ "Journal of the Geological Society," vol. xii., p. 251. 

 X " Acadian Geology," 2d ed., p. 26. 

 5 



