416 R. C. WALLACE 
in the Sailmohr the markings are vertical, rectilinear, and well 
defined. From the position and character of the markings, one 
may discard the theory that they are due to castings, which would 
in most cases be vertically disposed or grouped round definite 
centers. If taken simply as tracings, the difficulty in explaining 
the dolomitization still remains, and one is thrown back again 
on some such theory as the inclusion of water in the cavities left 
by the annelids. Although local dolomitization and mottling in 
limestone may in certain cases be attributed to annelids, one can 
hardly consider that markings of the kind found in the mottled 
limestones of Manitoba are due to this cause. 
There remains the hypothesis that the mottling is connected 
with algic decomposition.- Analyses due to Goedechens' show that 
the percentage of MgO in the ash.of algae collected from the west 
coast of Scotland may reach 11.66. If then the algae of the sea 
bottom become buried under a thin coating of calcareous ooze 
before actual decomposition ensued, the liberated Mg salts might, 
in conjunction with the sea water of fairly high Mg content, cause 
such increase of Mg ions locally as to give rise to actual dolomitiza- 
tion. Only from such organisms and allied types could the per- 
centage of Mg salts be increased locally to any appreciable extent. 
There are certain structural features of the markings that lend 
some support to this view of the origin of the dolomitization. 
They are horizontally placed, are markedly dendritic, and the sec- 
tions often show a darker core which might represent the actual 
position of the plant; while the magnesian waters, extending out- 
ward from this central nucleus, have affected the surrounding 
stone. Again, thin sections of the dolomitized areas occasionally 
show a narrow central tube of clear, well-crystallized calcite (see 
Fig. 6), indicating that a cavity had existed when the dolomitiza- 
tion took place, and that this was subsequently infilled with calcite. 
Such cavities might be formed when, owing to decomposition, the 
organism disappeared. The hematite and limonite of the recrys- 
tallized dolomitic material would be attributed to the iron salts 
of the algae. Fig. 7 shows a normal cross-section, where no sub- 
sequent infilling has taken place. 
t Pfeffer, Pflanzen physiologie, I, 110. 
