Dr. C. Callaway — Cheddar Gorge. 67 



angles; posterior extremity narrower and rounder. Dorsal margin 

 nearly straight, slightly incurved at the midiUe, where the anterior 

 flange tapers off. Ventral margin straight, but in the lateral aspect 

 encroached upon by the postero - ventral swelling. Surface of 

 carapace gradually rising from the antero-dorsal angle toward the 

 postero-veutral, where it is steep, and surmounts the posterior 

 flange ; somewhat pitted and having three or more small tubercles 

 arranged in the centre of each valve. Outline, as seen from above, 

 irregularly ovate, widest at the posterior third, the anterior ex- 

 tremity rapidly tapering and concave in that part of the outline; 

 posteriorly the carapace ends in a blunt point. End view sub- 

 orbicular and slightly wedge-shaped at the dorsal margin. 



Length, '76 mm. ; height, '5 mm. ; thickness of carapace, '53 mm. 



From the Alveolina - limestone, No. 3,335&. Farafra Oasis : 

 Libyan Series. 



[To b» tontinued : the Plates will he given in our next Number.) 



IV. — The Zigzag Course of the Cheddar Gorge. 

 By C. Callaway, M.A., D.Sc, F.G.S. 



HAVING undertaken to explain the geology of the Cheddar 

 gorge to the Cotteswold Naturalists' Field Club at one of 

 their field-meetings this Summer, I thought I should do so more 

 intelligently if I first acquired a practical knowledge of my subject. 

 I accordingly paid a preliminary visit to Cheddar. There were two 

 chief questions to be investigated : (1) the origin of the gorge, and 

 (2) the cause of its serpentine, or rather zigzag, course. The 

 former inquiry had been answered in four different ways. There 

 had been geologists who held that the valley had been excavated 

 by the waves of the sea. A second school taught that it had been 

 produced by a dislocation, splitting the rocks asunder. The third 

 theory regarded it as a valley of erosion, excavated in the ordinary 

 way by a river on the surface. A fourth explanation placed the 

 river underground, and maintained that the glen had originally been 

 a long winding cavern, the roof of which had subsequently fallen in.^ 



Very little investigation was needed to convince me that hypotheses 

 one and two were of no value. My inquiry led me to consider that 

 on the whole the cavern theory was most in accordance with the 

 facts, and I provisionally adopted it in my subsequent exposition 

 to the Club. I confess, however, that with my present knowledge 

 of the locality I am unable to find a decisive preponderance of 

 evidence in its favoui", and I leave the question still open. I therefore 

 limit myself in this paper to the second inquiry — the serpentine 

 course of the gorge. 



I felt some surprise to find that a stream running down so steep 

 a slope should have described such rapid curves. The floor of the 

 valley at its mouth, near where the underground river emerges 

 at the surface, is 85 feet ^ above sea-level. In half a mile it rises to 

 340 feet, and in a mile to 500 feet, or a rise of 255 feet in the first 

 half-mile and 160 feet in the second. Just beyond the mile, at 



^ See also Proc. Geol. Assoc, vol. xi, pp. ccvii and 493. 

 ^ The heights are taken from the 6 -inch Ordnance Map. 



