1864,] DAWSON STRUCTtJEE OF EOZOON. 51 



2, On the Structure of certain ORaAific Eemains in the Laurentian 

 Limestones of Canada. By J. W. Dawson, LL.D., F.R.S., F.G.S,, 

 Principal of M'Gill University, Montreal. 



[Plates VI. & VII.] 



At the request of Sir "William E. Logan, I have submitted to micro- 

 scopic examination slices of certain peculiar laminated forms, con- 

 sisting of alternate layers of carbonate of lime and serpentine, or of 

 carbonate of lime and white pyroxene, found in the Laurentian 

 Limestones of Canada, and regarded by Sir William as possibly 

 fossils*. I have also examined slices of a number of limestones and 

 serpentines from the Laurentian Series, not showing the external 

 forms of these supposed fossils. 



The slices were prepared by the lapidary of the Survey, and were 

 carefully examined under ordinary and polarized light, with objectives 

 made by Ross and Smith & Beck, and also with good French ob- 

 jectives. 



The specimens first mentioned are masses, often several inches in 

 diameter, presenting to the naked eye alternate laminae of serpentine, 

 or of pyroxene, and carbonate of lime. Their general aspect, as 

 remarked by Sir W. E, Logan (Geology of Canada, 1863, p. 49), 

 reminds the observer of that of the Silurian Corals of the genus 

 Stromatopora, except that the laminae diverge from and approach 

 each other, and frequently anastomose or are connected by transverse 

 septa. 



Under the microscope the resemblance to Stromatopora is seen 

 to be in general form merely, and no trace appears of the radiating 

 cells characteristic of that genus. The laminae of serpentine and 

 pyroxene present no organic structure, and the latter mineral is 

 highly crystalline. The laminae of carbonate of lime, on the con- 

 trary, retain distinct traces of structures which cannot be of a 

 crystalline or concretionary character. They constitute parallel or 

 concentric partitions of variable thickness, enclosing flattened spaces 

 or chambers frequently crossed by transverse plates or septa, in 

 some places so numerous as to give a vesicular appearance, in others 

 occurring only at rare intervals (PI. VI., PI. VII. fig. 1). The laminae 

 themselves are excavated on their sides into rounded pits, and are in 

 some places traversed by canals, or contain secondary rounded ceUs 

 apparently isolated (PI, VII. fig. 2). In addition to these general 

 appearances, the substance of the laminae, where most perfectly 

 preserved, is seen to present a fine granular structure, and to be 

 penetrated by numerous minute tubuli, which are arranged in 

 bundles of great beauty and complexity, diverging in sheaf-like 

 forms, and in their finer extensions anastomosing so as to form a 

 network (PL VII. figs. 3«, 4). In transverse sections and under 

 high powers, the tubuli are seen to be circular in outline and sharply 

 defined (PL VII. fig. 5). In longitudinal sections they sometimes 

 present a beaded or jointed appearance. Even where the tubular 



* Canadian Naturalist and Geologist, 1859, p. 49. 



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