30 Principal J. W. Dawson ow Eozoon canadense. 



tlie assertion that structures resembling the canal-system of 

 Eozoon may he seen in gneissose rocks. 



As applied to the ordinary serpentinous specimens, Hahn's 

 theory of their origin may be stated thus : — He agrees with 

 the advocates of the organic nature oi Eozoon in admitting 

 that the layers of calcite are an original part of the formation. 

 He supposes,"^owever, that the serpentine was originally oli- 

 vine, which, like serpentine, is a silicate of magnesia, but 

 anhydrous, and differing somewhat in the proportions of its 

 ingredients. The olivine by absorption of water became con- 

 verted into serpentine, and necessarily swelled to a greater bulk 

 than before*. This expansion caused it to force itself between 

 the layers of limestone and to assume a laminated form. The 

 limestone at the same time became softened and fissured ; and 

 its fissures or pores were injected with calcareous matters held 

 in solution or suspension in the water saturating the rock. 

 In this way the laminse and the canal-system are to be accounted 

 for. The "proper wall" he holds to be merely a film of 

 needles of chrysotile or fibrous serpentine surrounding the 

 grains and plates of that mineral. These views he supports 

 by statements grouped under the three heads of Geological, 

 Mineralogical, and Zoological Facts ; but the two former can- 

 not well be separated from each other, and the latter are, from 

 his point of view, of course altogether subordinate. For the 

 sake of clearness, I may arrange his arguments and my cri- 

 ticisms under the following heads. 



1. Preliminary Assumption. — Hahn informs us that he 

 " started from the proposition that for every part of a rock 

 the presumption is in favour of mere rock-formation." Surely 

 not, when a definite form visible to the naked eye is in ques- 

 tion. In the present case it was the resemblance of the masses 

 oi Eozoon to the familiar Stromatoporce of the Silurian which 

 first directed attention to them. The microscopist has a right 

 to inquire whether in such a case internal structure confirms 

 tlie indication of external form, but not to proceed from the 

 assumption of mineral origin, even when tlie microscope fails 

 to reveal structure. Further, when portions only of such a 

 specimen show organic structure, this is always held to afford 

 evidence of organic nature, even though these portions should 

 be small and exceptional. 



2. Geological Relations. — As to these, Hahn seems to be in 

 some doubt. He asks — "Are they [the serpentine nodules and 

 layers] merely imbedded in the limestone, and therefore formed 

 before it, or were they produced simultaneously?" and he 

 remarks, " This question can be decided only on the spot." It 



* The expansion would be about in the ratio of 4 to 3. 



