74 A. EF. Verrill—The Bermuda Islands; Geology. 
unevenly corroded or pitted upper surface gives good opportunities 
for the commencement of the formation of “ pot-holes,” large and 
small, by the scouring action of the sand and rushing waters. Prob- 
ably some of the deeper and narrower cups or pot-holes on the ser- 
puline atolls are directly due to remodelling and enlarging the cavities 
called molds of ‘palmetto stumps,” which often abound in this 
limestone much above sea level, as well as between tides. (See chap- 
ters 20 and 24, 0.) 
It seems evident to me that the vast number of durable flat reefs, 
littoral shelves, submerged benches, and serpuline atolls, all along 
the south coast, are due largely to the outcroppings of these hard, 
nearly horizontal limestone strata, which have just the right nature 
and position to easily yield such flat structures, by the erosive action 
of the waves. (See figures 11, 27-29.) But I do not wish to deny 
that similar structures can be, and often are carved from ordinary 
eeolian limestones, especially when the layers are horizontal or nearly 
sc, as Mr. Agassiz states. 
I am, therefore, inclined to believe that most of the serpuline 
atolls and outlying flat reefs of the south coast are composed of the 
hard limestones of the Walsingham period. But I do not know that 
the characteristic, extinct, fossil land snails have as yet been found 
in these reefs. As a rule, these solid limestones, in this vicinity, are 
destitute of recognizable fossils. 
b. Red Clay layers, with extinct Land Snails. 
That the Walsingham formation, which I refer to the Pliocene, 
represents a long period of time, is evident, not only on account of the 
great thickness of its limestone strata, but also from the successive 
layers of red-clay soil interstratified with them, as described above. 
Each of these layers of soil indicates a long time for surface decom- 
position, and locally without sand-drifting. Six or seven of these 
layers of soil, varying in thickness, have been noticed in some sec- 
tions; most of them are only 2 or 3 inches thick, but some are 8 to 10 
inches or more. 
It is thought that it would require the solution of at least 150 
feet of limestone to form a single foot of this soil, not allowing 
anything for that portion which would naturally be washed away 
by rain. (See chapter 20, A.) 
Prof. T. W. Goldie, in his printed lecture on the Geological 
Formation of Bermuda, 1893, pp. 14, 15, mentioned a “belt or layer” 
of “red clay” soil, 8 inches thick, underlying the xolian limestones 
