All this, however, has nothing to do with the layers of fragments of Eozoon which are scattered through the Laurentian limestones. In these the fossil is sometimes preserved in the ordinary manner, with its cavities filled with serpentine, and the thicker parts of the skeleton having their canals filled with this substance. In this case the chambers may have been occupied with serpentine before it was broken up. At St. Pierre there are distinct layers of this kind, from half an inch to several inches in thickness, regularly interstratified with the ordinary limestone. In other layers no serpentine occurs, but the interstices of the fragments are filled with crystalline dolomite or magnesian limestone, which has also penetrated the canals; and there are indications, though less manifest, that some at least of the layers of pure limestone are composed of fragmental Eozoon. [Illustration: Fig. 52.--_Acervuline Variety of Eozoon, Côte St. Pierre._ (_a_) General form, half natural size. (_b_) Portion of cellular interior, magnified, showing the course of the tubuli.] [Illustration: Fig. 53.--_Archæospherinæ from Côte St. Pierre._ (_a_) Specimens dissolved out by acid, the lower one showing interior septa. (_b_) Specimens seen in section.] In the Laurentian limestone of Wentworth, belonging apparently to the same band with that of St. Pierre, there are many small rounded pieces of limestone, evidently the debris of some older rock, broken up and rounded by attrition. In some of these fragments the structure of Eozoon may be plainly perceived. This shows that still older limestones composed of Eozoon were at that time undergoing waste, and carries our view of the existence of this fossil back to the very beginning of the Grenville series of the Laurentian. With respect to organic fragments not showing the structure of Eozoon, I have not as yet been able to refer these to any definite origin. Some of them may be simply thick portions of the shell of Eozoon with their pores filled with calcite, so as to present a homogeneous appearance. Others have much the appearance of fragments of such Primordial forms as _Archæocyathus_, now usually regarded as corals or sponges; but after much careful search, I have thus far been unable to say more than I could say in 1865.