116 THE DAWN OF LIFE. 



parts only ; since the calcareous skeleton is preserved, in most 

 cases, without any alteration. The vacant spaces lef d by the 

 decay of the sarcode may be supposed to have been filled by a 

 process of infiltration, in which the silicates were deposited 

 from solution in water, like the silica which fills up the pores 

 of wood in the process of silicification. The replacing sili- 

 cates, so far as yet observed, are a white pyroxene, a pale green 

 serpentine, and a dark green alumino-magnesian mineral, 

 which is allied in composition to chlorite and to pyrosclerite, 

 and which I have referred to loganite. The calcareous septa 

 in the last case are found to be dolomitic, but in the other in- 

 stances are nearly pure carbonate of lime. The relations of 

 the carbonate and the silicates are well seen in thin sections 

 under the microscope, especially by polarized light. The 

 calcite, dolomite, and pyroxene exhibit their crystalline struc- 

 ture to the unaided eye; and the serpentine and loganite are 

 also seen to be crystalline when examined with the microscope. 

 When portions of the fossil are submitted to the action of an 

 acid, the carbonate of lime is dissolved, and a coherent mass 

 of serpentine is obtained, which is a perfect cast of the soft 

 parts of the Eozoon. The form of the sarcode which filled 

 the chambers and cells is beautifully shown, as well as the 

 connecting canals and the groups of tubuli ; these latter are 

 seen in great perfection upon surfaces from which the carbon- 

 ate of lime has been partially dissolved. Their preservation 

 is generally most complete when the replacing mineral is ser- 

 pentine, although very perfect specimens are sometimes 

 found in pyroxene. The crystallization of the latter mineral 

 appears, however, in most cases to have disturbed the calca- 

 reous septa. 



*' Serpentine and pyroxene are generally associated in these 

 specimens, as if their disposition had marked difierent stages 

 of a contirmous process. At the Calumet, one specimen of the 

 fossil exhibits the whole of the sarcode replaced by serpen- 

 tine ; while, in another one from the same locality, a layer of 

 pale green translucent serpentine occurs in immediate contact 

 with the white pyroxene. The calcareous septa in this speci- 

 men are very thin, and are transverse to the plane of contact 



