In the tenth volume of this Magazine, a correspondence appeared upon the subject of “True and Apparent Dip,” which was started by Mr. Penning, and continued by several able geologists. The object of the present article is to call attention to another branch of the same subject. We are all acquainted with “Sopwith's Models,” in which are experimentally shown the outcrops, or “traces,” of plane strata upon variously curved surfaces. We now are about to refer to the outcrop of curved strata upon a plane surface. I was led to examine this question from the following circumstances.