had afterward been cracked by shrinkage, and the cracks filled with a second deposit of serpentine. Now, serpentine is a hydrous silicate of magnesia, and all that we need to suppose is that in the deposits of the Laurentian sea magnesia was present instead of iron and potash, and we can understand that the Laurentian fossil has been petrified by infiltration with serpentine, as more modern Foraminifera have been with glauconite, which, though it usually has little magnesia, often has a considerable percentage of alumina. Further, in specimens of Eozoon from Burgess, the filling mineral is loganite, a compound of silica, alumina, magnesia and iron, with water, and in certain Silurian limestones from New Brunswick and Wales, in which the delicate microscopic pores of the skeletons of stalked star-fishes or Crinoids have been filled with mineral deposits, so that when decalcified these are most beautifully represented by their casts, Dr. Hunt has proved the filling mineral to be a silicate of alumina, iron, magnesia and potash, intermediate between serpentine and glauconite. We have, therefore, ample warrant for adhering to Dr. Hunt's conclusion that the Laurentian serpentine was deposited under conditions similar to those of the modern green-sand. Indeed, independently of Eozoon, it is impossible that any geologist who has studied the manner in which this mineral is associated with the Laurentian limestones could believe it to have been formed in any other way. Nor need we be astonished at the fineness of the infiltration by which these minute tubes, perhaps 1/10000 of an inch in diameter, are filled with mineral matter. The micro-geologist well knows how, in more modern deposits, the finest pores of fossils are filled, and that mineral matter in solution can penetrate the smallest openings that the microscope can detect. Wherever the fluids of the living body can penetrate, there also mineral substances can be carried, and this natural injection, effected under great pressure and with the advantage of ample time, can surpass any of the feats of the anatomical manipulator. Fig. 25 represents a microscopic joint of a Crinoid from the Upper Silurian of New Brunswick, injected with the hydrous silicate already referred to, and fig. 26 shows a microscopic chambered or spiral shell, from a Welsh Silurian limestone, with its cavities filled with a similar substance. [Illustration: Fig. 25. _Joint of a Crinoid, having its pores injected with a Hydrous Silicate._ Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25 diameters.] [Illustration: Fig. 26. _Shell from a Silurian Limestone, Wales; its cavity filled with a Hydrous Silicate._ Magnified 25 diameters.]