Principal J, W. Dawson on Eozoon canadense. 35 



more perfect than where the skeleton is transparent cleava.ble 

 calcite ; and where the cleavage-planes become very distinct 

 the canal-system has apparently in some places been altogether 

 obliterated. Again, when the large trunks of the canals are 

 filled with serpentine, and the finer branches with Dolomite or 

 calcite, the serpentine sometimes ends abruptly, as if cut off. 

 In those beds which contain angular fragments oi Eozoon, the 

 canals of course end at the limits of such fragments. Cases 

 of these kinds account for this appearance in the instances in 

 which it is observed. But if the canals did happen, without 

 any such peculiar circumstances, to be limited by crystalline 

 forms, this would only be an example of a fact familiar to 

 every one experienced in examining fossils under the micro- 

 scope. I have now before me a slice of crinoidal Trenton 

 limestone in which the fragments of Crinoids show perfectly 

 their cellular structure ; but each fragment is inscribed in a 

 hexagonal or rhombic crystal of transparent calcite, so that 

 the structure may be said in every case to be limited by a 

 crystalline individual. I have another specimen of a crinoid 

 from the altered rocks of the White Mountains, in which each 

 joint has. the cleavage proper to a crystalline individual, and 

 the minute structures are preserved only in small spots here 

 and there. I have many specimens of calcified coniferous 

 wood from the Coal Formation in which the wdiole substance 

 consists of cleavable calcite crystals ; and yet in some portions 

 the structures are completely preserved, though in places they 

 end abruptly and mysteriously at the edges or in certain parts 

 of the length of crystalline individuals. I might cite many 

 other illustrations ; and such cases are familiar to micro- 

 scopists. 



As to the minute prismatic crystals of carbonate of lime 

 sometimes seen to be imbedded in the calcite of the skeleton 

 of Eozoon, and which Hahn regards as aragonite (though they 

 are certainly sometimes seen to be traversed by cleavage-planes 

 like those of calcite), these have no definite relation to the 

 canals, among or beside which they lie just as any other im- 

 bedded minerals would do. They are evidently merely portions 

 of the calcal-eous matter which for some reason have crystallized 

 differently from the rest ; and possibly in some cases proximity 

 to the canals may have been one determining cause of their 

 formation. 



7. The iuhdated Proper Wall. — This Hahn is content to 



acid. A practised eye can detect the peculiar granulation of the forami- 

 niferal skeleton even in fragments scattered through inorganic limestone 

 or Dolomite, and when the other structures may not be perceptible. 



3* 



