296 Mackintosh — Geological Notes on Devon. 



Mr. Pengelly, as to leave any other observer comparatively little to 

 say. Among his most important discoveries must be ranked that of 

 lithodomous perforations in limestone rocks at considerable altitudes 

 above the sea. 



Pholas-borings. — I have lately been hunting for these perforations, 

 which competent authorities regard as P^oZas-borings, and have seen 

 them in various positions and at different levels, but principally on the 

 summits and sides of Kent's Hill and Asheldon (see Plate XIII). The 

 extremely fresh appearance of the borings where they are only very 

 slightly protected by vegetation, and even where they are exposed 

 to the atmosphere, would seem, at first sight, to forbid our assign- 

 ing to them any great antiquity. But if we are to refer them to the 

 great glacial submergence, or (according to most geologists) a pre- 

 glacial submergence, then the preservation of these borings clearly 

 shows that the prevailing theory of the superficial (not internal) 

 dissolution of limestone hills by subaerial action is a mere assump- 

 tion. It may be asserted that the perforations have been preserved 

 only in the hard parts of rocks, the softer parts of which have dis- 

 appeared ; but the following facts render it certain that little or no 

 dissolution of the limestone has occurred since the perforations were 

 made, or since this locality was last under the sea. On the summits 

 and sides of the hills, and in the valleys, wherever natural or arti- 

 ficial exposures of rock occur, they exhibit undoubted wave-marks, 

 consisting of smoothed and rounded surfaces, grooves, cells, pot- 

 shaped cavities, etc., on which the Molluscan-borings have been im- 

 pressed, or by lohich previous borings have been modified, or par- 

 tially effaced. The most decided P^oZas-borings do not, so far as 

 I have observed, occur principally in clifis, but on the sloping sides 

 or summits of hills. The highest I have yet seen occur near the 

 summit of Kent's Hill, at an altitude of at least 240 feet above the 

 sea. Many apparent PJ^o/as-borings, especially those which occur on 

 the edges of limestone strata, or which are associated with a honey- 

 combed rock-surface, cannot be relied on. When on the face of a 

 solid mass, they may be more satisfactory ; but the sea is capable 

 (as may be seen on the rocky beach under the public baths at 

 Torquay) of forming holes of so many forms and sizes, by the vn- 

 equal disintegration of the rock, as well as by the gyratory move- 

 ment of silt and stones, that it is scarcely safe to assume that any 

 holes have been bored by organic agency unless they appear very 

 cleanly cut, and of a iiniform specific shape. The most decided 

 Pholas-horings I have seen, occur in colonies, and vary in the widest 

 part from half-an-inch to an inch and a quarter in diameter. 



There is a truly wonderful natural arch^ at Upton, near Torquay, 

 the general form of which impressively points to the former action 

 of the sea. There are numerous holes on the sides as well as on 

 the roof of this arch. Many of the smaller holes are not unlike 

 slightly weathered -Pi^oZas-borings, especially those which occur 



' Mr. Pengelly has more than once referred to this arch as a monument of sea- 

 action, and I noticed it in the Geol. Mag,, No. 2, Vol. III. Feb. 1866, p. 68. 



