286 Correspondence—Prof. T. G. Bonney. 
it strengthened my argument, which was to show that a respectable 
pebble was not easily made by running water, and because, in the 
case of the rocks with which I was dealing, the rounding mentioned 
by Mr. Irving is, as a rule, a very secondary and subordinate matter. 
(1.) Every one knows that certain rocks become tolerably rounded 
by mere aérial waste, but the débris which reaches Alpine torrents 
(in the districts of which I spoke) is commonly angular; and this 
is equally true of the material to which my inferences applied. I 
may add that I believe few things are more important in attempting 
to reason inductively from observed facts than to be careful in pre- 
serving a due relation between quantities of the first and second 
order of magnitude (as they are called by mathematicians). Over- 
much precision of statement and an elaborate parade of small details 
interfere with our sense of proportion, and there is great danger, if 
you look at a sprat for too long a time, and from too near a point of 
view, that you may at last fancy it a whale. 
(2.) In regard to “the scouring action of sand,” I cannot pretend 
to say how much is done by the knocking of the pebbles together, 
and how much by the friction of passing sand ; but I certainly cannot 
make the distinction which Mr. Irving attempts todo. All the rivers 
of which I spoke transport quantities of sand as well as pebbles in 
all parts of their course, though it is only in the lower part that they 
can deposit much of the former. However, as it takes a very long 
journey to remove the angles from a grain of sand, I have my doubts 
as to its conspicuous efficiency as a fashioner of pebbles out of pieces 
of hard rock, and if Mr. Irving alludes to the action of sand on 
pebbles which but rarely travel, then I think it would tend to flatten 
rather than to round them. 
(3.) In regard to the general question raised, viz. the origin of 
the pebbles and other materials of the Bunter group, space will not 
permit me to enter into details, so I must forbear to criticize minor 
but not unimportant points in Mr. Irving’s letter, such as ‘the 
pebble-beds proper being quite local,” a statement which is only true 
if a most liberally extended sense be given to the last word. But I 
express a fundamental dissent by asking whether there is any 
evidence at all in favour of the Bunter being a marine deposit, and 
still more a deposit in a sea where, according to the ordinary rules, 
strong coast currents or a rolling surf would be likely to exist. All 
I can say is that the Bunter as a whole is remarkably unlike every 
admitted marine formation which I have ever examined, while it 
presents a strong resemblance to such deposits as parts of the Old 
Red Sandstone, some beds in the Lower Carboniferous of Scotland, 
and the Nagelflue of the Alps: deposits, which most geologists agree 
in considering more or less fluviatile: nay, allowing for a slight 
difference in colour and hardness, the Bunter pebble-beds of Central 
England (I said nothing about Southern England) are indistinguish- 
able from many of the old sub-Alpine river-drifts which I have 
repeatedly examined. T. G. Bonney. 
