136 THE DAWN OP LIFE. 



their forms they miglit readily be mistaken for it ; and 

 associated with, them are broken pieces of other porous 

 organisms which may belong to Protozoa^ though this 

 is not yet certain. 



Of all the fossils of the Silurian rocks those 

 which most resemble Bozoon are the Stromato^oroe, 

 or ^' layer- corals/' whose resemblance to the old 

 Laurentian fossil at once struck Sir William Logan ; 

 and these occur in the earliest great oceanic lime- 

 stones which succeed the Primordial period, those 

 of the Trenton group_, in the Siluro-Cambrian. From 

 this they extend upward as far as the Devonian, ap- 

 pearing everywhere in the limestones, and themselves 

 often constituting large masses of calcareous rock. 

 Our figure (fig. 42) shows a small example of one of 

 these fossils; and when sawn asunder or broken 

 across and weathered, they precisely resemble Eozoon 

 in general appearance, especially when, as sometimes 

 happens, their cell-walls have been silicified. 



There are, however, difi'erent types of these fossils. 

 The most common, the Stromatoporse properly so 

 called, consist of concentric layers of calcareous matter 

 attached to each other by pillar-like processes, which, 

 as well as the layers, are made up of little threads of 

 limestone netted together, or radiating from the tops 

 and bottoms of the pillars, and forming a very porous 

 substance. Though they have been regarded as corals 

 by some, they are more generally believed to be Proto- 

 zoa ; but whether more nearly allied to sponges or to 

 Foraminifera may admit of doubt. Some of the more 



