OF CENTRAL CANADA PART IV. 



215 



FIG. 118. 

 Recent Diatoms. 

 Greatly magnified. 



and recent age consist almost entirely of these minute forms. The 

 well-known "Tripoli," used as a polishing material, is of this charac- 

 ter. Diatornaceous deposits were for- 

 merly called " infusorial marls," di- 

 atoms having been at first regarded as 

 animal infusoria. Fig. 118 shews a few 

 of these forms, highly magnified. In 

 Canada, diatomaceous beds are all but 

 unknown. The only recorded example 

 (Rep. Geol. Survey, 1863) is in the val- 

 ley of the Petewahweh, in the Upper 

 Ottawa region ; but a bed of this nature 

 is said to occur at Westbury in Cornpton 

 County, Quebec. Thin sections of chert nodules from our corniferous 

 (Devonian) limestone, have also shewn the presence of diatoms. 



Acrogens : Whilst in Thallogens, the plant, so to say, is practi- 

 cally all leaf or all stem, and growth takes place from no definite 

 point, in acrogens there is a distinct differentiation of stem and leaf, 

 and the growth is acrogenous or terminal. The division falls into 

 two subdivisions : Cellular Acrogens and Vascular Acrogens. 



In cellular acrogens, the plant, as in the thallogens, is composed of 

 cellular tissue only. The sub-division comprises : Characece, Mosses, 

 and Hepaticacece. Fossil representatives of these (with the exception 

 of the " nucules," of certain charse) are exceedingly rare, and of no 

 special interest. The nucules of charse (fresh-water plants) are 

 minute seed-like organs, encased in five spirally-twisted filaments, the 



many feet ; or otherwise the creature must have moved forward by a series of spasmodic jerks 

 or jumps, alighting always in an exact line with the end of the trail, so as to avoid the slightest 

 overlap or other break of symmetry in the entire impression. Any other mode of progression 

 would unavoidably have effaced or smudged the transverse grooves or ridges as the bod}- of 

 the animal passed over them. There is also another point which appears to be in complete 

 opposition to the assumed track-origin of these impressions. In places, two, or even three, of 

 these supposed tracks cross one another, but at the crossing points there is no sign of disturb- 

 ance or smearing, so to say, such as must inevitably have occurred if one trail had been carried 

 across another. As shown especially in Sir William Logan's original figure, representing a 

 group of several "tracks" (Geol. of Canada, 1863, p. 107), the one impression simply conceals 

 or lies over the other at these points, as would happen if two fucoid -fronds, or other similar 

 bodies, were drifted together to a sandy shore, and were there covered simultaneously with 

 sediment. 



In attributing these impressions to large fucoids, we encounter, on the other hand, no real 

 difficulty. Many algae, it is well known, present transverse furrows ; and a salient example of 

 this character may be seen in our Arthrophycus Harlani, so abundant in many of the Medina 

 and Clinton beds. 



