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formed primarily by solution action. Such holes are most numerous in soluble rocks, notably limestone. They may almost invariably be distinguished from other potholes by the rough solution surfaces of their interiors.

Class B, Type 2:

Dentpits as described by Hudson22 have been previously mentioned as vortex formed, shallow concavities on rather pure calcareous rock where the water carries but little matter in mechanical suspension. They are due mainly to solution and their width greatly exceeds their depth. The diameters of

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FIG. 5. VIEW OF FRACTURED END OF THE LOWER PORTION OF THE BLOCK SHOWN IN FIG. 4. Presents a side view of the cupholes and illustrates parabolic vertical sections of the same. Shows also the edge of the dent-pitted surface below and indicates approximately the quantitative value of the two forms of erosion as geological agents.

dentpits are usually between 1 and 5 centimeters. In vertical sections they present rather circular outlines. No silt is ever found in these depressions which are well shown in Fig. 7.

Class C, Type 1:

Tide pools are a rather uncommon type of pothole depression occurring along rocky coast lines. Their formation seems 22 Hudson, G. H., "Joint Caves of Valcour Island-Their Age and Origin," N. Y. State Mus. Bull., No. 140, 6th Rept. of the Director, 1909, pp. 165-173, 1910.

to be due to a variety of processes. A particular occurrence of such holes along the west coast of Vancouver Island, near Port Renfrew, B. C., is described and explained by Henkel.23 The principal rock formations at this locality are, in their order from the surface downward, sandstone, conglomerate and shale. In many places the sandstone has been worn away leaving the conglomerate or the shale as the surface rock. Depressions of varying shape and depth are found in all three kinds of rock but are best developed in the sandstone because of the "extreme softness of this rock. The pools are also numerous in the shale, but only few occur in the conglomerate. In the sandstone the pools generally occur in strata that are nearly horizontal and they are especially numerous in a gently dipping

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Photo by G. H. Hudson, Plattsburg, N. Y.

FIG. 6. JOINT WELLS IN LIMESTONE LEDGE ON VALCOUR ISLAND.

sandstone which contains many concretions.

This formation

is locally known as the "devil's billiard table." Apparently

23 Henkel, Isabel, "A Study of Tide Pools on the West Coast of Vancouver Island," Postelsia-The Year Book of the Minnesota Seaside Station, 1906, pp. 277, 304.

the concretions have been worn out in many places leaving a depression. The number, size and irregularity of the pools increases with the slope.

The majority of the smaller pools are circular but the larger ones are generally elliptical and give evidence of having been formed by the union of two or more smaller pools. From depressions a few inches in diameter, the size varies to those 30 feet long, 20 feet wide and 10 to 15 feet deep. Plant and animal life is commonly found in many of the pools, notably sea urchins, mussels and barnacles, and in some pools boulders occur that are apparently of the same composition as the concretions. The larger pools, however, contain the fewer boulders.

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FIG. 7. THE UNDERSURFACE OF BLOCK SHOWN IN FIGS. 4 AND 5, SHOWING DENT PITS. Figure should be viewed at an angle that will eliminate the impression of botryoidal surface.

Henkel considers the occurrence of the concretions to be the most potent factor in the development of sandstone pools. After becoming detached from the formation the concretions

are hurled about inside the depressions gradually enlarging them while at the same time the concretions are reduced in size. This process of enlargement may be aided by plant and animal life exerting a disrupting influence upon the rock. Tide pools in general, however, according to this author are initiated by a variety of conditions and processes, among which may be included: cracks in the rock; concretions; lines of stratification; erosion by waves, tides and wind; action of carbon dioxide; variation in temperature, both of water and of the air; and the action of plants and animals.

Class C, Type 2:

Potholes due to sea urchins are depressions occurring along rocky coast lines and are a type very closely resembling Tide Pools. Such potholes are described by Fewkes24 who states that they are comparatively rare. Some were found near Grand Manan, New Brunswick, and also at Biarritz, France. They are apparently most commonly found on coasts beaten by a strong sea and in places where there is considerable tidal variation. It seems that the cavities owe their origin primarily to sea urchins, the teeth and spines of which gradually hollow out the depression. Water action rolling these spiny creatures about, and pebbles also apparently facilitates the development of such pools.

(To be concluded)

24 Fewkes, J. W., "On Excavations made in Rocks by Sea Urchins," The American Naturalist, Vol. 24, pp. 1-21, January, 1890.

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recently elected permanent secretary of the Paris Academy of Sciences in succession to the late Jean Gaston Darboux. Both M. Picard and M. Darboux have been professors of mathematics and have been active in matters of national defense and in increasing the services to the nation of the Paris Academy of Science. Like M. Painleve the present premier of France, also professor of mathematics, their activities seem to disprove the prevalent idea that mathematicians are not efficient in practical affairs.

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