438 Proceedings of Royal Society of Edinburgh. [sess. 
forms of lake-gytje, viz., Diatom-gytje, Cyanophycea-gytje, and 
Chitin-gytje. The first-named, which occurs mostly in the colder 
lakes, contains enormous quantities of plankton Diatom frustules, 
and may consist almost exclusively of these ; skeletons of bottom 
Diatoms are very rare. From gytjes of this composition the 
Diatom clay may arise. According to Forel it seems that the 
Diatom skeletons in deeper lake-bottoms may he dissolved and 
disappear, but this is not the case in our shallower lakes. The 
Cyanophycea-gytje is a black, fetid substance, consisting of decaying 
plankton Cyanophycea, and mostly occurs in warm shallow lakes. 
The Chitin-gytje contains enormous quantities of the valves of 
Daphnids, and is generally formed in small lakes devoid of 
Cyanophycea. Lately, Holmboe (1903) has found Diatom-gytje as 
well as Chitin-gytje fossil in Norwegian peat-moors. 
The constituents of the lake-gytje are not the same all over the 
lake-floor, notable differences being recognisable on the two sides 
of the 30-feet contour-line. Outside this contour we hardly ever 
find stems, shells, and Mollusca (except Pisidium), and very 
seldom leaves, the deposit nearly always consisting of fine mud. 
On the other hand, inside the 30-feet contour we often find the 
whole bottom strewn with shells ; leaves and stems are common, and 
the deposit is much coarser in texture, often containing considerable 
quantities of sand and gravel, which are rarely found outside the 
30-feet contour. As already stated, the material inside the 30-feet 
contour is either deposited, and forms, for example, peat, or is, sooner 
or later, pulverised by the action of the waves dashing it against 
the stones and sandy bays of the beach ; hand in hand with this 
mechanical action a chemical process goes on, especially as regards 
the lime deposits. A close study of the mollusc shells from the 
shore and shallow water shows a very conspicuous corrosion, caused 
by different factors. On this point I may refer to my bottom 
explorations (1901, p. 152), and would here only observe that 
hitherto the corrosion of the shells of living animals has been 
studied chiefly as a conchological curiosity, without full appreciation 
of the fact that the corroding influences are nature’s principal 
instruments in the pulverisation and dissolution of lime secreted 
by organisms The process of pulverisation and dissolution of all 
the waste material inside the 30-feet contour is greatly accelerated 
