would seem that the lower and older part of the fossil had been wholly converted into serpentine or pyroxene, or had so nearly experienced this change that only small parts of the calcareous wall can be recognised. These portions correspond with fossil woods altogether silicified, not only by the filling of the cells, but also by the conversion of the walls into silica. I have specimens which manifestly show the transition from the ordinary condition of filling with serpentine to one in which the cell-walls are represented obscurely by one shade of this mineral and the cavities by another. The above considerations as to mode of preservation of Eozoon concur with those in previous chapters in showing its oceanic character; but the ocean of the Eozoic period may not have been so deep as at present, and its waters were probably warm and well stocked with mineral matters derived from the newly formed land, or from hot springs in its own bottom. On this point the interesting investigations of Dr. Hunt with reference to the chemical conditions of the Silurian seas, allow us to suppose that the Laurentian ocean may have been much more richly stored, more especially with salts of lime and magnesia, than that of subsequent times. Hence the conditions of warmth, light, and nutriment, required by such gigantic Protozoans would all be present, and hence, also no doubt, some of the peculiarities of its mineralization. NOTES TO CHAPTER V. (A.) Dr. Sterry Hunt on the Mineralogy of Eozoon and the containing Rocks. It was fortunate for the recognition of Eozoon that Dr. Hunt had, before its discovery, made so thorough researches into the chemistry of the Laurentian series, and was prepared to show the chemical possibilities of the preservation of fossils in these ancient deposits. The following able summary of his views was appended to the original description of the fossil in the _Journal of the Geological Society_.