1 84 The American Geologist. September, i90i. 
CaCO.j. Some of the limestone of this type in Petoskey, Mich., 
and presumably of the same horizon, is reported to yield ioo% 
CaCO... The purity of these limestones is readily accounted 
for when we remember that the reef-building- organisms flour- 
ish only in water free from terrigenous matter. Since the 
stratified limestone beds which always accompany the reef are 
derived from the organically formed limestone, and since there 
is an absence of terrigenous matter within the area of their 
deposition, it follows that these limestones must be very pure, 
within a radius of some magnitude from the central coral reef, 
the source of supply of the lime sand. It furthermore fol- 
lows that the thickness of the coral reef, and the thickness of 
the flanking fragmental limestones, may agree and that beds 
of great thickness may accumulate, depending on the length of 
time during which the reef remains in an actively growing con- 
dition. 
It thus appears that the reefs are the most trustworthy 
guides to the purity of the limestones. Close to the reefs from 
which they were formed, these limestones will generally be free 
from foreign material, while this may increase in amount pro- 
gressively with the distance from the reef. 
Loc. p. Quarry of Alpena Cement Co. This limestone is 
gray and crystalline, and heavv bedded showing- for the most 
part perfect stratification. The coral reef character is shown in 
several parts of the quarry, its structure being very similar to 
the reef of the preceding locality. Large masses of Favosites 
and Acervularia make up the greater part of the reef, and 
with these occur large stromatoporoids. Among the associated 
remains are Gypidida rouiingcri, Atrypa reticularis. Spirifer 
mucronatus, Pterinea Hahella and others. 
This reef, though similar to the one of Loc. i, appears 
to be entirely distinct from it. The two are, however, con- 
nected by sparingly fossiliferous coral sand, which was derived 
from both, and constitutes tlie main material of the bedded 
limestones. 
Loc. la. The rock exposed here shows essentially the same 
characteristics as that of the preceding two localities. Favo- 
sites appears to be the most common fossil. Other fossils are 
Atrypa reticularis, Spirifer granulosus, Zaphrentis sp. and sev- 
eral species of Stropheoiloiita. 
