238 THOMAS C. BROWN 
structures of the conglomerate bed, but also to the minute micro- 
scopic structure of the individual pebbles, has brought out some 
interesting features not hitherto noted in the descriptions. It has 
already been stated that these peculiar conglomerate pebbles have 
a wide range in size and shape and that they occur associated with 
odlite grains. Although the pebbles are broad and flat, the edges 
and corners are always rounded. ‘Thin sections cut from these 
pebbles show a peculiar banding in the broad flat types and a con- 
centric structure in the smaller rounded specimens (Figs. 1, 2). 
The pebbles are frequently partially or completely replaced by 
silica in the form of chert. In such cases the matrix weathers 
away and leaves the replaced pebbles like chert concretions. If 
the replacement is incomplete and a joint plane passes through the 
pebble, the central unreplaced part also weathers out, leaving the 
chert nodule hollow. 
Fic. 1.—Cross-section of a broad flat pebble as it appears on a weathered surface. 
The weathering brings out the arrangement of the laminae. Natural size. 
As an outcome of these studies it has been concluded that these 
structures are organic, resulting from the activities of calcareous 
algae. This conclusion differs from Seeley’s in assigning the origin 
to algae instead of sponges. Seeley was undoubtedly led to assign 
these structures to the sponges because of the frequency with which 
he found them containing silica. He describes Wingia as “‘a cal- 
careous fossil sponge, with or without siliceous spicules . . . . the 
essential structure a collection of pillae or tufted balls, these mostly 
0.25 mm. in diameter, massed without definite arrangement or 
rarely more loosely distributed through the containing calcite.” 
These siliceous structures are no doubt replacements of the original 
calcite similar to those found both in the pebbles and in the odlites 
of the Pennsylvania region. 
The importance of the lime-secreting algae seems to have been 
generally overlooked in explaining the origin of early Paleozoic lime- 
stones. Even in the building-up of modern calcareous deposits 
