Professor Hutfcon, in the eighth of his lectures on Geology, in the Museum Hall, on Monday evening, treated upon the grouping of the vari ous rocks, before a good audience. It had been shown -in previous lectures that during a large portion of the earth's existence, rocks had been forming, and that attempts had been made to classify them accoi ding to their age. Ihere was no means of ascertaining their actual age, but there were many ways of testing their relative ages. It was shown that there were two kinds of rocks — eruptive and sedimentary. The latter were in regular beds, and the former in irre gular masses. Geologists took the stratified rocks as the foundation of a chronological scale, as their ages could be made out with greater certainty than those of the eruptive rocks, and to that scale the totter were referred. The best test, when it could be applied, of the rela tive age of sedimentary rocks, was the super position of one on another ; for it was evident from their mode of origin, that tbe upper bed must be younger than the one upon which ifc rested. But this was not always correct, when one part of a bed was compared with a distant part of another bed. A bed of eand might be forming along a coast line ; outside of this a bed of clay might be in process of deposition ; while further out still a stratum of limestone or chalk might be accumulating. If the land was slowly sinking, the shore line would recede, and the inner eds;e of all these formations would follow ifc ; so that the old sand-bed would be covered with clay, and the clay by limestone or ch.dk. So that iv time we should have a stratum of sandstone covered by clay, which would be again overlaid by limestone ; and yet different parts of all these three b> ds formed contemporane ously. This «as made clear by means of a dia gram. Another evidence of the relative ages of two rocks was when fragments o£ two rocks were found included in the other. The mineial composition of rocks was also cf great use in determining their age, but it could not always be relied upon. The most valuable evidence, however, was found in the fossils contained in rocks. 