every variety of Laurentian limestone, of altered limestones from the Cambrian and Silurian, and of serpentine marbles of all the varieties furnished by our collections. These were examined with ordinary and polarized light, and with every variety of illumination. Dr. Hunt, on his part, undertook the chemical investigation of the various associated minerals. An extensive series of notes and camera tracings were made of all the appearances observed; and of some of the more important structures beautiful drawings were executed by the late Mr. H. S. Smith, the then palæontological draughtsman of the Survey. The result of the whole investigation was a firm conviction that the structure was organic and probably foraminiferal, and that it could be distinguished from any merely mineral or crystalline forms occurring in these or other limestones. At this stage of the matter, and after exhibiting to Sir William all the characteristic appearances in comparison with such concretionary, dendritic, and crystalline structures as most resembled them, and also with the structure of recent and fossil Foraminifera, I suggested that the further prosecution of the matter should be handed over to Mr. Billings, as palæontologist of the Survey, and as our highest authority on the fossils of the older rocks. [Illustration: Fig. 30.--_Casts of Canals of Eozoon, in Serpentine._ Decalcified and highly magnified.] [Illustration: Fig. 31.--_Group of finest Tubuli._ Highly magnified, from a micro-photograph.]