THE ORIGIN OF CERTAIN PALEOZOIC SEDIMENTS 245 
ments of the spherical grains from that locality where they seem 
to be largest show that the range for the diameters is between 
1.00 mm. and 1.33 mm. Many of the grains are partially or 
completely replaced by silica and a rounded sand grain frequently 
forms the nucleus around which these odlites have formed. In the 
course of this replacement the central nuclear sand grain is some- 
times secondarily enlarged, showing a zonal structure in optical 
Fic. 5.—Photomicrograph of siliceous odlite; 22. The odlite on the left shows 
an oval sand grain as a nucleus, surrounded by cryptocrystalline quartz, with a nar- 
row zone of minute quartz crystals radially arranged around the outer margin. The 
upper odlite shows the central nuclear quartz grain secondarily enlarged into a quartz 
crystal with part of the crystal faces developed. The lower oélite shows the central 
space occupied by an aggregate of minute quartz crystals. Nicols crossed. 
Fic. 6.—The interior of the odlite of Fig. 5, 130, showing the crystal outlines 
of the minute quartz grains which compose it. Nicols crossed. 
continuity with the original grain and sometimes with more or less 
completely developed crystal faces. The outer zones of the spherules 
are generally replaced by cryptocrystalline quartz or chert. When 
no nucleus is present this chert may extend to the center or the center 
may be occupied by numerous distinctly crystallized quartz grains. 
Some of these show crystal faces as if the minute central cavity had 
been filled like a tiny geode. Another very interesting feature is the 
secondary enlargement of the spherules by one or more zones of crys- 
talline quartz deposited in minute radial crystals arranged in zones 
around the original granule, with the long axes of the crystals 
arranged normal to the outer surface of the sphere (Figs. 5, 6). 
