RECENT AND FOSSIL RIPPLE-MARK. 
7 
Moulds made in this way require a much shorter time to harden 
than when made under water. 
For making moulds under water too deep to use the wooden 
frame referred to above I have devised a piece of apparatus 
comprising a base about 16 inches in diameter resembling an 
inverted dish-pan with a sharp edge, in which the plaster mould 
is formed. A cylindrical tin reservoir holding about 2| quarts 
is mounted on top of the broad circular base with which it is 
connected by a wide short neck at the bottom. This neck fits 
snugly into a hole in the centre of the base and is filled with a 
valve opening upward. The top of the reservoir is closed by a 
cap like that of a milk can. A small hole in the centre of this 
cap permits a string to pass which is connected with the valve 
at the bottom of the reservoir, thus permitting the operator to 
open the valve after the apparatus is in position for taking a 
mould on the bottom. Two or more valves in the top of the 
lower section of the apparatus so constructed as to open in 
descending and automatically close on reaching bottom facilitate 
the process of lowering to the bottom. Short nails inserted 
through holes in the lower margin of the apparatus prevent the 
mould from slipping out when the apparatus is lifted. The 
reservoir is filled with a mixture of plaster of paris and water, 
equal volumes of each being used; after reaching bottom the 
valve at the bottom of the reservoir is opened by means of the 
attached cord and the plaster of paris mixture pours into the 
base of the apparatus and forms the desired mould. It is neces- 
sary to empty the reservoir within five or six minutes after 
preparing the mixture since it becomes viscid in six or eight 
minutes. During the hour required for the setting of the plaster 
the apparatus is left marked by a buoy. In using either this or 
the ripple-mark mould apparatus the operator should be pro- 
vided with a small instrument known as a “telegraph snapper” 
for sampling the bottom and locating sandy deposits in which 
ripple-mark may be expected. Another desirable but more 
expensive piece of apparatus which can be used advantageously 
in studying current ripple-mark is the Ekman current metre. 
This instrument records not only the velocity but the direction 
of the current as well. It, therefore, enables the investigator 
