PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 407 
as if the origin might be due to seaweed remains. Often the colored portion 
approaches the color of yellow ochre, and seéms strongly impregnated with 
iron, while the intervening spaces are more or less colored. 
Later work on the Galena and Trenton of Iowa by Leonard’ 
has shown that the Galena, which is considered to be a dolomitized 
phase of the Trenton, is found to grade into the underlying Trenton 
through strata which possess a somewhat similar mottled appear- 
ance to those already described. An analysis showed that the 
darker areas were dolomitized, while the lighter were unaffected 
by the magnesia-bearing waters. This was inferred from the 
chemical analyses for MgCO, and CaCO, which were as follows: 
grey portion, 97.46 per cent CaCo,, 4.31 per cent MgCO,; buff 
portion, 60.97 per cent CaCO,, 18.28 per cent MgCO, (op. cit., 
p. 259). It was also suggested that jointing planes and the spaces 
between the larger fossils and the surrounding limestone might 
have served as chambers of passage for the waters which effected 
the dolomitization. It was observed that although the fossils 
were not themselves affected, they were frequently surrounded 
by a dolomitized area. 
Under the microscope the difference in structure between the 
two areas is very apparent (Fig. 5). The darker-colored patches 
are evenly crystallized, showing sections of rhombohedra of dolo- 
mite, set close together, and occasional crystals of hematite passing 
over into limonite. The color is due to the hematite and, more 
particularly, the limonite, to which the action of percolating water 
has imparted a banded structure. Excepting a few large shells, 
which have not been affected, there are no traces of organic remains 
in the dolomitized areas (see Fig. 4). The light-colored areas, 
on the other hand, contain numerous fragments of brachiopod 
shells, with occasional sections of polyzoa and corals. These are 
set in a fine-grained calcitic material, strikingly different, even at 
the margin between the two areas, from the fairly perfectly crystal- 
lized dolomite. No hematite or limonite occurs in the lighter 
material except where occasional local dolomitization has taken 
place, and there the rhombohedra are always colored by the iron 
ore. That this is actually a case of pseudobrecciation, and not a 
t “Geology of Clayton County,” Jowa Geological Survey, XVI, 250. 
