THE GEOLOGY OF SWITZERLAND. 
37 
organic matter which it contains. Heer suggests that 
the best explanation may be afforded by the Sar- 
gasso Sea. The Atlantic Ocean, for an area of about 
40,000 square miles, is covered by Sargasso-weed so 
densely that ships sometimes find a difficulty in 
forcing their way through it. The sea is deep, and 
the fragments of dead weed are probably quite de- 
cayed before they reach the bottom, to which they 
would give a dark colour. He thus explains the colour 
of this limestone. 
The “Btlndner Schiefer” so largely developed in 
the Grisons and Valais are now considered, from the 
fossils which have been discovered in several places, 
to belong to this period. It it probable however that 
the strata marked as Btlndner Schiefer on the Swiss 
maps do not all belong to the same period. 
Dogger or Brown Jura. 
Switzerland was for the most part under water 
at this period, but that there must have been land 
in the neighbourhood during some part of the time 
is proved by the existence, near Porrentruy, of beds 
containing several species of Limpets (Patella), Peri- 
winkles (Purpura), Mussels (Mytilus), Neritas, and 
other shore molluscs. It is probable that the Black 
Forest and the Vosges were then dry land. 
