410 R. C. WALLACE 
strip of dolomitic material—the dolomitization having ceased when 
the percolating waters met round the shell. That breccia of sucha 
type could have existed without the fossils becoming detached is 
hardly possible. A study of the margin of the darker areas leaves 
no doubt that we are dealing with a secondary dolomitization. 
Although there is a definite marginal line, it shows so sharp inter- 
penetration of dolomitized and undolomitized material that it 
could have been caused only by the irregular advance of waters 
bearing magnesian salts in solution. 
Microscopic investigation shows that the hematite, which with 
the limonite is responsible for the color effect of the darker patches, 
is found in crystals separate from the dolomite, and at the edges 
of the dolomite rhombohedra. The dolomite is itself clear and 
colorless; and the position of the hematite crystals, and their invari- 
able association with dolomitic areas suggest naturally that the 
hematite was formed by the same agency that gave rise to the 
dolomitization. From the microscopic evidence alone, the most 
feasible explanation seemed to be that during the process of dolo- 
mitization the ferruginous material, originally present in the form 
of ferrous carbonate as an isomorphous admixture with the calcitic 
material, had to a large extent separated out owing to the greater 
inability of the dolomite to hold the iron isomorphously. Once 
separated as carbonate, the iron would undergo oxidation much 
more rapidly than is possible in a mixed crystal where the calcite 
exercises a controlling influence, retaining the iron in the ferrous 
state. A chemical analysis should then show—if this theory can 
be supported—that although the darker areas are richer in magnesia 
and in ferric iron, and the lighter areas contain practically no mag- 
nesia, the total iron is approximately the same in both. 
CHEMICAL INVESTIGATION 
The two varieties were separated as completely as possible, and 
subjected to chemical analysis. The calcium was estimated 
volumetrically, the magnesium gravimetrically. The total iron is 
given here as Fe,O;. The iron precipitate was dissolved and 
reduced by zinc, and the iron estimated volumetrically. The 
ferrous iron was estimated volumetrically in a separate portion. 
