A Michigan Preliminary Section. — Grabaii. 183 
The origin of these Hmestone masses and the accompanying 
beds of stratified hmestone may be explained as follows : 
In the Devonic sea of this region, the luxuriant growth of 
corals and other lime-secreting organisms produced isolated 
reefs, which rose to within the sphere of wave activity. Being 
subject to the continued attack of the waves, these reefs were 
destroyed wherever the vitality of the poh'ps was insufficient 
to resist the wave attack. Wherever exposed, the dead coral 
rock was ground into a coral sand, this being accomplished in 
part by the direct activity of the waves, in part hy the aid of 
tools chiefly in the form of loose blocks which were rolled about, 
and served to grind up tlu? coral rock, and in part by the many 
reef-destroying organisms, which in every reef are actively 
breaking up the dead coral masses. The resulting coral sand 
was carried away by the waves and currents, and deposited on 
the flanks of the reefs, and in the quieter water beyond. 
Stratification is well marked in such a fraginental limestone, 
or lime-sandstone, and it not infrequently -happens that cross- 
bedding structure, ripplemarks, and other shallow water char- 
acteristics are shown in such a limestone. In fact, we may 
consider that there is.no essential difference in structure be- 
tween such lime-sandstones and ordinary quartz-sandstones, 
the mineralogical character of the component grains being the 
only distinction. In such a lime-sandstone fossils need not 
be abundant, in fact we could understand their total absence. 
Near the growing reef, which is the source of the coral sand, 
the minor reef organisms may be expected to occur, but their 
number would decrease in proportion as we pass away from the 
reef. In the immediate vicinity of the reef, an interlocking 
of the organically and the mechanically formed limestones 
occurs, for at times the coral sand encroaches upon the reef, 
and again the reef organisms extend outward, growing on the 
foundation of sedimentary coral sand. This inter-locking of 
the two types of limestone is well shown in the quarries opened 
in the reefs in question, and on either side the stratified lime- 
stone, consisting wholly of consolidated coral sand, is seen dip- 
ping away from the reef. This stratified liiuestone is striking- 
ly barren of organic remains ; only a few brachiopods or small 
corals being found at intervals. In texture it is very imiform, 
and in composition very pure, yielding upward of g/7o of 
