1865.] LOGAN — LAURENTIAN FOSSILS. 97 



One of the specimens had been sliced and submitted to micro- 

 scopic observation, but unfortunately it was one of those composed 

 of loganite and dolomite. In these, the minute structure is rarely 

 seen. The true character of the specimens thus remained in 

 suspense until last winter, when I accidentally observed indications 

 of similar forms in blocks of Laurentian limestone which had been 

 brought to our museum by Mr. James Lowe, one of our explorers, 

 to be sawn up for marble. In this case the forms were composed 

 of serpentine and calc-spar ; and slices of them having been pre- 

 pared for the microscope, the minute structure was observed in 

 the first one submitted to inspection. At the request of Mr. 

 Billings, the palaeontologist of our Survey, the specimens were 

 confided for examination and description to Dr. J. W. Dawson, of 

 Montreal, our most practised observer with the microscope ; and 

 the conclusions at which he has arrived are appended to this com- 

 munication He finds that the serpentine, which was supposed to 

 replace the organic form, really fills the interspaces of the calca- 

 reous fossil. This exhibits in some parts a well-preserved organic 

 structure, which Dr. Dawson describes as that of a Foraminifer, 

 growing in large sessile patches after the manner of Polytrema and 

 Carpentaria, but of much larger dimensions, and presenting 

 minute points which reveal a structure resembling that of other 

 Foraniiniferal forms, as, for example, Calcarinci and Nummulina. 



Dr. Dawson's description is accompanied by some remarks by 

 Dr. S terry Hunt on the mineralogical relations of the fossil. He 

 observes that while the calcareous septa which form the skeleton of 

 the Foraminifer in general remain unchanged, the sarcode has 

 been replaced by certain silicates which have not only filled up the 

 chambers, cells, and septal orifices, but have been injected into the 

 minute tubuli, which are thus perfectly preserved, as may be seen 

 by removing the calcareous matter by. an acid. The replacing 

 silicates are white pyroxene, serpentine, loganite, and pyrallolite or 

 rensselaerite. The pyroxene and serpentine are often found in 

 contact, filling contiguous chambers in the fossil, and were evi- 

 dently formed in consecutive stages of a continuous process. In 

 the Burgess specimens, while the sarcode is replaced by loganite, 

 the calcareous skeleton, as has already been stated, has been re- 

 placed by dolomite, and the finer parts of the structure have been 

 almost wholly obliterated. But in the other specimens, where the 

 skeleton still preserves its calcareous character, the resemblance 

 between the mode of preservation of the ancient Laurentian For- 



Vol. II. q No. 2. 



