We give no opinion regarding the theories of Professor Mitchill, not intending to review the work, but merely to aid, as far as in our power, in drawing the public attention to the interesting subjects about which it is occupied. If we have any remark to add, it is, that an adherence to the technical precision with which most rocks are at the present day described, appears desirable in mineralogical and geological descriptions. When in the valuable additions before us we read of schorl rock, we gain only the idea of a rock containing that mineral; but as it occurs occasionally in several of the primitive rocks, we are at a loss which is intended; we believe it never forms a rock by itself. So with the slate rocks: there are several varieties of them--mica slate, clay slate, greenstone slate, &c. besides some subdivisions; and the mere word slate does not always give us the precise idea. But we are aware that, in the present case, it was less in view to go into all the details of geological description, than to give a view of our organized remains and of their supposed origin. ART. XII. _Notice of Eaton's Index to the Geology of the Northern States, together with a Transverse Section of the Catskill Mountain to the Atlantic._ The extensive collection of facts in this little book of fifty-four pages, is creditable to the author's industry and discernment: he informs us that he has travelled 1000 miles on foot, while investigating the geology of the district concerning which he has written. This district is certainly interesting, and every attempt to diffuse correct information concerning it, deserves encouragement. Mr. Eaton's account of the regions he has explored, has every mark of verisimilitude; and we commend his efforts to diffuse geological information, by short courses of lectures, in different towns. In his arrangement of rocks, he has deviated from Werner--has adopted some views of Bakewell, and some of his own. Werner's arrangement of rocks has, undoubtedly, its imperfections and its redundancies; and yet it may be questioned how far his system has been really improved by its different emendators. If Werner, by mentioning argillaceous schistus only in the primitive class of rocks, left us to dispose of it where we might, when we find it at one time, covering or sustaining anthracite, with impressions of ferns, and at another with impressions of fish and vegetables, and in contact with bituminous coal; still those who, with Mr. Eaton, throw argillaceous slate into the transition class, and omit it in the primitive and secondary, embarrass us with an equal difficulty; for we find argillaceous slate in contact, and alternating with, mica slate, and without any impressions of organized bodies, when we must, without a doubt, call it primitive.