No. 3.] SPENCRR — PALAEOZOIC GEOLOGY. 159 



•disintegrated fragments, which are assuming the more crystal- 

 line condition have their interspaces filled with carbonate of 

 lime dissolved in the water, which was probably derived from 

 the orijj:inal material of the shells. 



If the organic remains be included in a matrix of the same 

 color, not only the form but also the certainty of its former pres- 

 ence in any position is apt to be lost. P]specially is this the case 

 with the corals and shells which are composed of aragonite. 

 However, if the surfaces of the organisnis were covered by thin 

 layers of some foreign matter, as pyrites or mud, the former may 

 still be preserved, but the place occupied by the structure will 

 be found to have a more highly crystalline structure than the 

 matrix itself, as the carbonate of lime of the shells, not having a 

 great surface exposed by being broken into fragments, has more 

 time for gradual re-arrangement of molecules, and, consequently, 

 larger and more perfect crystalline forms are produced. This is 

 found to be particularly the case with Lamellibrauchiate shells 

 (aragonite) in the rocks of the Niagara group at Hamilton, 

 where only the remains of casts, procured in the manner just 

 described are to be found, although some beds indicate that they 

 were originally made up of a mass of these shells. The best pre- 

 served frairments of organic structure in our rocks are stems of 

 erinoids, but these are generally re-crystallized, although they 

 were even at first in the forms of small crystals of calcite. 



The corals generally have become silicified but the forms are 

 so far changed as to show that the original calcareous matter was 

 re-crystallized before its replacement with silica was accom- 

 plished. 



Some of the Graptolites are well preserved owing to the large 

 amount of corn(;ous matter that may have arrested molecular 

 change. From obscure casts some of the beds of limestones 

 appear to have been derived from Orthocerata. Brachiopods 

 are the commonest fossils retaining any of their original appear- 

 ance. Polyzoa are fairly preserved, especially in the " Chert 

 bed," where also a few Gasteropoda retain their calcareous 

 structure. In fact nearly all the fossils arc better preserved in 

 the " Chert bed" than elsewhere. This fact may in some way 

 be accounted for owing to the presence of soluble silica derived 

 from the sponges having cemented the calcareous plates together 

 at the time when the animal matter of the structures was being 

 gradually removed, for many of the fossils seem to be saturated 

 with siliceous material. 



