THE ORIGIN OF CERTAIN PALEOZOIC SEDIMENTS 239 
these forms have been given scant credit. A recent contribution 
on this subject’ by Howe has indicated how important these 
organisms really are in building up the reefs of today. The lack 
of recognition of these forms as important reef-builders is no doubt 
due to two facts. In the first place, the skeletons of these calcare- 
ous algae either have been assigned to true corals and therefore 
credited to the animal kingdom, or, on the other hand, they have 
perhaps been considered by some, direct chemical precipitates 
rather than organic structures. In the second place, these calcare- 
ous algal skeletons lose their organic character with great rapidity. 
Walther has shown by his studies on the Lithothamnion bank in 
the Bay of Naples that by the 
action of the percolating water 
the Lithothamnion structure is 
gradually obliterated and the 
calcareous mass becomes a struc- 
tureless limestone. In studying 
Nullipore chalk from the Terti- 
ary and from the Lias, Walther 
found that in many parts there 
occurred well-preserved  speci- 
mens but in other parts a grad- 
ual obliteration is observed of all 
plant structures until the rock 
becomes entirely structureless. 
If this be true for a form like Lithothamnion which is known to 
secrete a calcareous skeleton whose mineral composition is calcite, 
it is not surprising that all trace of organic structure is lost in 
other types which, like Halimeda, secrete their skeleton in the 
mineral form of aragonite.’ 
It is freely admitted that in these pebble-like structures from 
the Cambrian and Ordovician limestones no organic structure 
has been found sufficiently well preserved to prove conclusively 
that they are of algal origin, but their similarity to such structures 
now forming is very suggestive. In this connection it is worth 
Fic. 2.—Thin section of small pebbles 
showing concentric structure. Enlarged. 
1M. A. Howe, “The Building of ‘Coral’ Reefs,” Science, May 31, 1912. 
2W. Meigan, Centralblatt fiir Min., Geol., und Pal. (1901), pp. 577, 578. 
