50 THE DAWN OF LIFE. 



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 accident- 

 ally observed indications of similar forms in blocks of Lauren- 

 tian 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 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 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 

 calcareous fossil. This exhibits in some parts a well-preserved 

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

 Foraminif er, growing in large sessile patches after the manner 

 of Polytrema and Carpenteria, but of much larger dimensions, 

 and presenting minute points which reveal a structure re- 

 sembling that of other Foraminiferal forms, as, for example 

 Calcarina and Nummulina. 



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

 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 

 sarcode has been replaced by certain silicates which have not 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, serpen- 

 tine, loganite, and pyrallolite or rensselaerite. The pyroxene 

 and serpentine are often found in contact, filling contiguous 

 chambers in the fossil, and were evidently formed in consecu- 

 tive stages of a continuous process. In the Burgess specimens, 

 while the sarcode is replaced by loganite, the calcareous skele- 

 ton, as has already been stated, has been replaced by dolomite. 



