Sir W. Dawson— The Animal Nature of Eozoon. 503 



papers on the similarity of the mode of occurrence of silicified 

 Stromatoporae in the great dolomite of the Niagara formation with 

 that of Eozoon in the Grenville Limestone, in which dolomite 

 occurs in beds, in thin layers, and in disseminated crystals, in a 

 manner to show that it was an original constituent of the deposit. 

 Dolomite is also one of the most common minerals filling the 

 cavities of Eozoon, and especially the finer tubuli. The mode of 

 its occurrence on the small scale may be seen in the following 

 description of a section of a portion of a bed of limestone from 

 Cote St. Pierre, examined under a lens, after being treated with 

 dilute acid. The specimen comprised about six inches of the 

 thickness of the bed : — 

 Crystalline limestone with crystals of dolomite, constituting about 



one half (fragments of Eozoon in calcite portion).^ 

 More finely crystalline limestone, with rounded granules of serpen- 

 tine, some of them apparently moulded in cavities of ArchEBO- 



spherinte, or of chamberlets of Eozoon. 

 Limestone with dolomite as above, but including a thin layer of 



limestone with granules of serpentine. 

 Limestone and dolomite, with a few grains of serpentine and 



fragments of Eozoon. 

 Crystalline dolomite with a few fragments of Eozoon, as limestone, 



with canals in dolomite. 

 Limestone with fragments of Eozoon, granules of serpentine, and 



groups of chamberlets filled with serpentine. 

 We have thus a bed of limestone in which dolomitic and serpen- 

 tinous layers appear to alternate, and occasional fragments of Eozoon 

 occur in both, while the smaller forms resembling fossils are, so far 

 as can be observed, limited to the serpentinous layers. 



At Aruprion on the Ottawa a portion of the Grenville Limestone 

 presents dark graphitic layers parallel to the bedding, and giving it 

 a banded grey and white appearance which has led to its use* as 

 a marble. An analysis by Dr. Harrington shows that the graphitic 

 layers contain 8-32 per cent, of magnesia, the lighter layei's only 

 2-57 per cent., in the state of grains or crystals of dolomite. Asso- 

 ciated with the marble there are also beds of brown-weathering 

 dolomite, affording 42-10 of magnesia. The graphite in this marble, 

 under the microscope appears as fihrils and groups of minute clots, 

 and sometimes coats the surfaces of crystals or fragments of calcite, 

 the appeai-ances being not unlike those seen in carbonaceous and 

 bituminous limestones of later date. 



In both the above cases the magnesium carbonate is evidently 

 an original ingredient of the bed, and cannot have been introduced by 

 any metamorphic action. It must be explicable by the causes which 

 produce dolomite in more recent limestones. 



Dana has thrown light on these by his observations on the 

 occurrence of dolomite in the elevated coral island of Matea in 

 Polynesia,^ under circumstances which show that it was formed 



1 Distinguished by tlieir fine granular texture and canal-systems. 



2 a Corals and Coral Islands," p. 356, etc. 



