( 476 ) 
he could derive from the present height of the Groningen Dollard- 
polders, a sinking of 8 mm. a year. If it was allowed to apply that 
cypher to a longer period, the sinking would amount to 2—2,2 m., 
in the time from 1545 (the year of the first embankment) till 1850 
and of 8 -9 meters in 1000 years. This estimation exceeds so much 
the estimations derived from other data that for this very reason it 
appears to be unacceptable. 
We bear in mind, however, that Yenema’s values also comprise 
the sinking of the ground of the Dollard-polders in consequence of 
the shrinking of the clay and peat layers during those 200—300 
years. Beneath the clay the rests of the layer of peat remain, of which 
the ground consisted in earlier times and on which the Dollard*clay 
has settled. How thick these rests are (probably 1—2 in.) and to 
how much the shrinking in consequence of the deposited clay has 
amounted, is unknown, and so no estimation is possible. 
In 1872 Dr. G. A. van Geytenbeek l ) made a study of the situation 
of the layer of peat or of “derrie” (clayey peat) under the clay 
and of the . layer of clay in the Zealand polders. He found that 
the upper surface of the layer of “derrie” lay at about 2 m. 
below A.P. From the height of the surface of the polders diked in 
between the 10 th and 19 th century with regard to the open sea, 
he derives a sinking of the surface of 2,865 m., so on an average 
2,9 mm. a year. 
The Insp.-Gen. A. A. Bekaar has (according to the paper by Mr. 
J. C. Ramaer, Chief.-Engineer, made this calculation from the data 
of two hundred polders, and found a sinking of the surface of 1. 
so about equally large. This sinking, however, cannot be considered 
as a sinking of the ground, it is only a sinking of the sur ace 
of the polders. Two factors come into play, first the s ^ rin 
of the clay by compression, secondly the actual lowering o ^ 
ground: From the height of the polders during the success j^ 
embankments from the eleventh century up to the present t * me 
amount of this shrinking may be roughly estimated; probab y* ^ 
1 m. On this supposition the actual lowering of the ground in a 
would have amounted to only 0,8 m., so not quite one meter. 
The old remains on the beach at Domburg (namely the tel ^^ 
Nehellennia, coffins of a cemetary and many dwellings) lay ^ 
the range of dunes in earlier times. Now they become visib e on ^ 
beach, and have often been visible since 1680, when the win 
tinues to blow from the East or Northeast for a considera e 
b Proeve eener Geologische Verh. over de prov. Zeeland gedurende bet 
daagsche tijdvak. 
