Dr. Dawson's ^Dairn of Life.' 369 



of serpentine, and crystalline aggregations of other mineral 

 silicates, the removed substance being replaced by calcite or 

 some other mineral carbonate. In short, it is our firm con- 

 viction that the calcite or dolomite in which the " canal- 

 system " is imbedded, and which forms " the intermediate 

 skeleton," is the product of a process similar to that of pseu- 

 domorphism among minerals. In the case of rocks, however 

 no false form is assumed by the replacing mineral, as in pseu- 

 domorphic crystals. It is for this reason that one of us has 

 termed the process methylosis by which substances consti- 

 tuting rocks are changed into, or replaced by, others *. 



But although by our theory portions and aggregations of 

 mineral silicates have been externally removed, thereby pro- 

 ducing every variety of configuration known as the " canal- 

 system of Eozoon canadense^'' there is no reason why, under 

 certain conditions, the configurations themselves may not have 

 been totally or partially removed from their calcareous matrix, 

 and a cast of them left formed of a mineral carbonate. This 

 replacement would be strictly parallel to pseudomorphism in 

 its typical character : garnets converted into calcite are cha- 

 racteristic examples t- 



Reverting to the two new Eozoonal cases lately brought 

 forward, it must be admitted that they are illustrated in the 

 * Dawn of Life ' more definitely than those previously made 

 known ; still the description of them (pp. 92, 182) is extremely 

 meagre. It would be advantageous in this discussion to know, 

 not only more about " the structures in very great perfection " 

 seen " in slices," but to have some account of the same " stiiic 

 tures " observed " in decalcified specimens." Besides, the 

 fact of their occurring under abnormal circumstances (which 

 by-the-by is what usually happens in " canals filled with dolo- 

 mite "), as admitted by Dawson, tends materially to strengthen 

 our suspicion as to their nature. 



Now, the six " large tubes in dolomite " remind us strongly 

 of the four rods, once serpentine, but now composed of calcite, 

 represented in aur recent paper ''On the Serpentinite of the 

 Lizard "I; therefore, in the absence of precise information, 

 it may be suggested that as the latter are still in the shape of 



* 'Geological Magazine,' January 1872. 



t We have made known in Skye ophite examples of isolated grains, 

 some once malacolite, others loganite, imbedded in calcite, having their 

 crust remaining, but their interior filled with calcite. In many instances 

 the crust is more or less removed; in some it is singularly pitted, re- 

 minding one of the siliceous case of certain I'olycystines. See Proc. Roy. 

 Irish Acad. vol. i. ser. 2, p. 138. 



\ 'Philosophical Magazine,' I87fi, vol. i. pi. 2. fig. 11. 



Ann. & Ma(j. N. Hist. Ser. 4. Vol. xvii. 2o 



