Correspondence. 389 
any kind. It must hence be distinguished from vesicular slag which 
often is to be found floating on the sea on our coasts. 
How do these rock fragments float in spite of the fact that they 
we,igh as much as 0.8 grams a piece and have a specific gravity of 
nearly three times that of the water? 
In seeking an answer to this cjuestion the following facts are of im- 
portance. I noticed that small gaseous bubbles adhered to the under 
side of the fragments. On some of the fragments on shore I saw a 
loose coating of these bubbles. At the very edge of the water there 
were to be seen also fragments that evidently had just begun to float 
and were lifted by the gentle swell of the water, bouyed up by the 
bubbles, then to be caught by the current and to be carried away or 
again tossed on shore. With ebb and flood the shore line is kept 
moving and a greater number of fragments are thus given a chance to 
be launched on the water. I regret having been prevented from un- 
dertaking a more thorough investigation of all the conditions con- 
nected with the phenomenon, being occupied with other work. Nor 
had I the apparatus needed to collect samples of the gas which 
gathered under the fragments. 
It appears probable that the floating chips were provided not only 
with the large gas bubbles that were apparent to the unaided eye, but 
that they were really covered with a continuous film of gas. Such a 
tilm should prevent the water from adhering to the slate, and thus keep 
the latter afloat, as it would, so to speak, curve up from under the edge 
of the chip and cause the weight of the volume of water displaced to 
equal, or exceed, that of the rock fragment itself. Vegetable matter 
may also, to a certain extent, prevent the -adhesion of water. A small 
quantity of diatoms and other algous remains may yet be found adher- 
ing to the material. 
Singular as may appear the observations here related, I suppose we 
saw in this case a phenomenon, which may be c^uite common in places 
favorably situated on the seashore. Geologically, it may have a 
greater significance than one is inclined to ascribe to it at first sight. 
So.ME Cretaceous Drift Peb3le.s in Northern Iowa. 
As bearing on the question of the probable extent of distribution of 
the Cretaceous deposits under the drift in Minnesota, it may be worth 
the while to note the occurrence of Cretaceous materials in the drift at 
more southern places. On this I happened to make some observa- 
tions in eastern Iowa last summer. In the course of a study of different 
phases of the drift in Scott, Muscatine, Louisa, and Lee counties, I had 
occasion to examine closely several thousands of pebbles taken at more 
than half a hundred different localities. One pebble in about each 
thousand in what is known as the Kansan drift consists of a gray cal- 
careous soft shale containing Globigerina, Textularia, and cycloid 
fish scales. I have found these pebbles in each of the counties men- 
tioned. They seem to be generally present. From my experience this 
summer I would infer that in a lot of 1,000 pebbles collected nromi'c- 
