102 THE DAWN OF LIFE. 



the microscope shows minute needle-like parallel pro- 

 cesses representing its still finer tubuli. It is evident 

 that to have filled these tubuli the serpentine must 

 have been introduced in a state of actual solution, 

 and must have carried with it no foreign impurities. 

 Consequently we find that in the chambers themselves 

 the serpentine is pure ; and if we examine it under 

 polarized light, we see that it presents a singularly 

 curdled or irregularly laminated appearance, which I 

 have designated under the name septariiform, as if 

 it had an imperfectly crystalline structure, and had 

 been deposited in irregular laminse, beginning at the 

 sides of the chambers, and filling them toward the 

 middle, and had afterward been cracked by shrinkage, 

 and the cracks filled with a second deposit of serpen- 

 tine. Now, serpentine is a hydrous silicate of mag- 

 nesia, and all that we need to suppose is that in the 

 deposits of the Laurentian sea magnesia was present 

 instead of iron and potash, and we can understand 

 that the Laurentian fossil has been petrified by infil- 

 tration with serpentine, as more modern Foraminifera 

 have been with glauconite, which, though it usually 

 has little magnesia, often has a considerable percent- 

 age of alumina. Further, in specimens of Eozoon 

 from Burgess, the filling mineral is loganite, a com- 

 pound of silica, alumina, magnesia and iron, with 

 water, and in certain Silurian limestones from New 

 Brunswick and Wales, in which the delicate micro- 

 scopic pores of the skeletons of stalked star-fishes or 

 Crinoids have been filled with mineral deposits, so 



