It is obvious, that in a given series of beds, which are either horizontal or more or less obliquely inclined, the underlying strata were the first formed, and the upper beds were laid down afterwards. If, however, we trusted solely to the order of superposition in estimating relative age, our conclusions would sometimes be very far from the truth. Recent geological investigations have brought to light facts well nigh incredible as to the magnitude and extent of rock-foldings. In regions of great earth-movements, the crust has been broken along certain lines, and great masses of strata have been thrust for miles along the tops of newer rocks. Thus it may be brought about that the natural sequence of a set of beds has been entirely altered, and older rocks have come to overlie sediments of a later geological age. Facts such as these clearly illustrate the difficulties of correct geological interpretation. In the horizontal section (Fig. 2), from the summit of Büzistock on the left to Saasterg on the right, we have a striking case of intense rock-folding and dislocation[46]. Prof. Heim[47] of Geneva has given numerous illustrations of the almost incredible positions assumed in the Swiss Mountains by vast thicknesses of rocks, and in the accompanying section taken from a recent work by Rothpletz we have a compact example of the possibilities of earth-movements as an agent of rock-folding. The section illustrates very clearly an exception to the rule that the order of superposition of a set of beds indicates the relative age of the strata. The horizontal line at the base is drawn at a height of 1650 metres above sea-level, and the summit of Büzistock reaches a height of 2340 m. The youngest rocks seen in the diagram are the Eocene beds _e_, at the base and as small isolated patches on the right-hand end of the section; the main mass of material composing the higher ground has been bodily thrust over the Eocene rocks, and in this process some of the beds, _b_ and _c_, have been folded repeatedly on themselves. Similar instances of the overthrusting of a considerable thickness of strata have been described in the North-west Highlands of Scotland[48] and elsewhere in the British Isles. It is important therefore to draw attention to cases of extreme folding, as such phenomena are by no means exceptional in many parts of the world. [Illustration: FIG. 2. Section from Büzistock to Saasterg. [After Rothpletz, (94) Pl. II. fig. 2.] _a_ = Sernifit or Verrucano (Permian). _b_ = Röthidolomit etc. (Permian). _c_ = Dogger (Jurassic). _d_ = Malm (Jurassic). _e_ = Eocene.]