THE ORIGIN OF CERTAIN PALEOZOIC SEDIMENTS — 235 
Ordovician age. In addition to the sandy and shaly layers and the 
normal limestones, several beds of odlitic rock are also present. 
In these the odlites are comparatively large in size and at many 
horizons have been replaced by silica giving rise to the siliceous 
odlites of this region which are represented in almost every geologi- 
cal collection.’ 
Beds of the peculiar thin pebble conglomerate already described 
under the Cambrian also occur here and, in fact, make up a half 
or more of the first few hundred feet of the Beekmantown lime- 
stone. These conglomerate layers vary somewhat in character. 
In every case examined the pebbles consist of extremely fine- 
grained calcareous material and, except on weathered surfaces, the 
conglomerate character cannot as a rule be easily recognized. On 
a fractured surface the interior of the pebble-like structure is almost 
identical in appearance with the surrounding matrix. On a 
weathered surface these structures show a different, generally 
lighter, color and are easily recognized. They are exceedingly 
variable in size and shape. In some beds they are round or oval 
and when viewed in the hand specimen have the characteristic 
appearance of water-worn pebbles. They are generally thinner in 
one direction and in thin section often show a peculiar concentric 
banding following parallel to the broader sides. Occasionally a 
pebble can be found with a fossil shell inclosed. In the majority 
of the beds the pebbles are broad and flat, ranging up to three or 
four inches in diameter and generally less than half an inch thick. 
They are found in all positions in the beds, some lying flat parallel 
to the bedding planes and some even standing on edge. Under 
certain conditions these broad flat pebbles have been replaced 
either in part or completely by silica in the form of chert, while the 
surrounding matrix remains calcareous. When only partially 
replaced, the replacing silica forms shell-like layers around the 
exterior of the structure, while the central part remains calcareous 
and unchanged. 
«For published descriptions of these see V. Ziegler, Am. Jour. Sci., 4th ser., 
XXXIV (1912), 113-27; E. S. Moore, Jour. Geol., XX (1912), 259-69; G. R. Wieland, 
Am. Jour. Sci., IV (1897), 262-64; J. S. Diller, Bull. U.S.G.S. No. 150 (1898), pp. 
95-97; E. O. Hovey, Bull. Geol. Soc. Am., V (1893) 627-29; E. H. Barbour and J. 
Torrey, Am. Jour. Sci., XL (1890), 246-49. 
