1864.] LOGAN LAUEENTIAX FOSSILS. 49 



and calcspar ; and slices of them having been prepared for the micro- 

 scope, 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 de- 

 scription 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 communication. He finds that the serpen- 

 tine, which was supposed to replace the organic form, really fills the 

 interspaces of the calcareous 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 man- 

 ner of Polytrema and Garpenteria, but of much larger dimensions, 

 and presenting minute points which reveal a structure resembling 

 that of other Foraminiferal forms, as, for example, Calcarina and 

 Nummulina. Dr. Dawson's desci;iption is accomj)anied by some re- 

 marks by Dr. Sterry 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 sar- 

 code has been replaced by certain silicates which have not only filled 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 tubuJi, 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 Lauren tian Forami- 

 nifera and that of the allied forms in Tertiary and Recent deposits 

 (which, as Ehrenberg, Bailey, and Pourtales have shown, are injected 

 ■with glauconite) is obvious. 



The Grenville specimens belong to the highest of the three already 

 mentioned zones of Laurentian limestone, and it has not yet been 

 ascertained whether the fossil extends to the two conformable lower 

 ones, or to the calcareous zones of the overlying unconformable Upper 

 Laurentian series. It has not yet either been determined what re- 

 lation the strata from which the Burgess and Grand Calumet spe^ 

 cimens have been obtained bear to the Grenville limestone or to 

 one another. The zone of Grenville limestone is in some places 

 about 1500 feet thick, and it appears to be divided for considerable 

 distances into two or three parts by very thick bands of gneiss. 

 One of these occupies a position towards the lower part of the 

 limestone, and may have a volume of between 100 and 200 feet. 

 It is at the base of the limestone that the fossil occurs. This 

 part of the zone is largely composed of great and small irregular 

 masses of white crystalKne pyroxene, some of them twenty yards in 



VOL. XXT. PART I. E 



