Correspondence—Ur. A. R. Hunt. 477 
—too modest. A large number of forms found outside Florida 
are figured and described. In many respects Dr. Dall’s work may 
be regarded as a species of text-book on the Tertiary Mollusca of 
the United States, having especial reference to Florida. In several 
instances all the known fossils of a genus occurring in America 
are alluded-to under the description of that genus; and a good 
deal of critical revision is effected in almost every branch. It 
is the work of a thorough master of the subject, and we look 
forward with considerable interest to the appearance of Part III. in 
which the author hopes to begin and conclude the description of the 
bivalves, and to give a general summary and tables. Caplieete 
CORRESPONDENCE. 
THE PEBBLE-RIDGE AT WESTWARD HO. 
Srr,—The Pebble Ridge at Westward Ho has often been noticed 
by geologists, and was the subject of a paper by Mr. Pengelly, 
F.R.S., so long ago as 1868.1 During a recent visit my attention 
was attracted to a peculiarity in the “pebbles,” which, although 
not referred to by Mr. Pengelly, can scarcely have escaped observa- 
tion. So far as 1am aware, however, the said peculiarity has not 
been recorded. The feature referred to is the concentric lamination 
often induced in “pebbles” above a few inches in diameter. An 
outside coat about 4; of an inch in thickness peels off, and often 
underneath the first a second layer comes away. ‘Thus the “pebbles” 
exhibit all the appearance of a concentric concretionary structure. 
But as the “pebbles” can be traced to their origin in rhomboidal 
or rectangular blocks of the local grits, the concentric structure, 
following the varying curves of the rounded “pebbles,” must 
necessarily be induced and not original. Although this structure 
is very common I could not find a single instance in a “pebble” 
sufficiently small and light to carry away in my pocket. It is con- 
fined entirely to weighty stones. It would almost seem as though 
we had in these rounded “pebbles” a sort of pseudo-concentric 
cleavage caused by percussion, and if so we may possibly find in 
these “pebbles” a clue to the concentric concretionary structure 
often noticed by geologists, and which in certain cases may be due 
not so much to crystallization as to pressure acting equably on the 
surface of a nodule enclosed in softer strata. However, my object 
in writing is to call attention to a significant fact rather than to 
attempt to explain it. A. R. Hunt. 
Torquay. 
THE RELATIVE AGE OF FLINTS. 
Srr,—I have to thank Mr. Jukes-Browne for his article on the 
Relative Age of Flints in your July issue. His paper is a valuable 
contribution, and will materially assist the workers in this most 
difficult subject by directing their attention to those points which 
most require elucidation. 
I gather that, as regards tabular flint, he believes it was formed in 
1 Trans. Dey. Assoc. 1868, p. 419. 
