20 Rev. 0. Fisher — Oblique and Orthogonal Sections. 



sponding posterior oiitliue is more evenly concave. The posterior 

 side is compressed into a nearly straight ridge, which runs along the 

 concave margin of the bone. 



EXPLANATION OF PLATE I. 

 Fig. la. — Internal surface of scapula of Ornithocheints ; the figure is placed at the 

 proximal end, towards the coracoid margin. 

 ,, lb. — Superior margin of the same specimen. 



,, 2a. — Lateral aspect of lower jaw of Ornithocheirus xyphorhynchus. 

 ,, 2b. — Palatal aspect of same specimen, showing tooth-sockets. 

 ,, 3a. — Anterior termination of snout of Ornithocheirus Reedii, seen from the 



front. 

 ,, 35. — Lateral aspect of same specimen, showing (p) (C\) fragments of teeth. 

 All figures of the natural size. 



III. — Oblique and Orthogonal Sections of a Folded Plane. 

 By the Eev. 0. Fisher, M.A., F.G.S. 



N 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. 



Professor Prestwich, a few weeks ago, was so kind as to show me 

 a very typical instance of the superficial deposit which I call " trail," 

 in a railway cutting now in progress of construction near Chevening, 

 in Kent. The cutting is in Gault, and the trail appears as coarse 

 subangular flint gravel, unstratified, impacted in a brownish-red 

 matrix of sandy clay, which is very similar in composition to the 

 " clay with flints," which generally covers at a far higher level the 

 upper parts of the North Downs. This trail, as seen in the E. and 

 W. railway section, showed pockets sometimes four or five feet 

 deep, and having a general oblique trend all in one direction. 

 The natural conclusion, on a cursory examination, was that some 

 superficial horizontal pressure acting from W. towards E. had 

 given them this uniformity of trend. But upon viewing the sections 

 on the south bank from the north, it appeared that the trend 

 was exactly opposite to that of the sections on the north bank 

 when viewed from the south. This fact, at first sight, seemed 

 anomalous. But after a little consideration Professor Prestwich 

 suggested to me the true explanation. The oblique trend is not 

 necessarily real, but apparent only ; and is caused by the obliquity 

 between the direction of the furrow of trail and of the inclined 

 plane which cuts it. 



We had a model made in wood, and found that a furrow, whose 

 orthogonal section was semicircular, gave an obliquely placed loop 

 upon the face of the cutting plane. Of course this might have been 

 foreseen ; because it is well known that the oblique section of a 



