248 Barbour and Torrey — Microscopic Structure of Oolite. 



the specimens, pulling the spherules apart, displacing and dis- 

 torting them in an interesting manner. 



One of our most suggestive specimens of this oolite — an 

 intermediate form between the lime and the siliceous varieties 

 — shows two distinct bands, marked by a strong dividing line 

 line (see fig. 4). 



4. 



Fig. 3. — Portion from which the micro-section of fig. 4 was cut, at a. Fig. 4.— 

 Pennsylvania Oolite, Lso, Lime-Silica Oolite ; Slo, Silica-Lime Oolite magnified ten 

 diameters. 



Analysis of Lime-Silica Oolite (Lso). Analysis of Silica-Lime Oolite (Slo). 



Silica -. 3-70 Silica... 56-50 



Oxide of aluminium .. ) . Oxide of aluminium .. ) , r „ 



Oxide of iron .... \ Oxide of iron. \ 



Calcium carbonate 88*71 Calcium carbonate 16*84 



Magnesium carbonate. 8*09 Magnesium carbonate. 2*60 



Water 12*54 



100*92 



Specific gravity 2*654 99"38 



Specific gravity 2*688 



One band is a lime-silica oolite, the other, a silica-lime oolite. 

 The microscopic section of this piece shows the two bands 

 plainly (see fig. 4). The lime-silica band is distinguished by 

 spherules having a marked radiate structure. Crossing the 

 dark dividing line of granules, we find the silica-lime band 

 characterized by a lighter color, and by spherules having the 

 concentric structure, interspersed with occasional radiate strag- 

 glers. While there are indications of a different mode qf 

 formation, it seems probable that the siliceous oolite is derived 

 from the calcareous by the replacement of lime particles by 

 silica. 



