DESCRIPTION OF THE OOLITES 591 



opaque zone indicate that all these inner zones are of the same size. Like- 

 wise, the outer spheres are of equal size in all the oolites. These facts 

 point to uniform conditions during the periods of growth of the oolites. 

 The oolites are composed of silica and are Avithout a Aisible nucleus, 

 unless the assumption is made that the smaller oolites acted as nuclei. 

 This may have occurred in some of the oolites. They have a faintly 

 granular appearance under the higher power of the microscope. Grains 

 of sand preserve their uniform spacing in the oolites as in the shale, and 

 may occur anywhere in the oolite (plate 21, figure 3). There is no evi- 

 dence that they were pushed aside during the growth of the oolite, and 

 their position indicates that they have not acted as its nucleus. This is 

 of interest, for most oolites are usually described as having a nucleus. 

 This, however, is not essential, as oolites may develop without a nucleus. 

 No cross is produced under crossed nicols, Avhich is usually the case when 

 the material is chalcedony, as these oolites appear to he. Two oolites may 

 occur so close together that they have interfered with each other's growth 

 on the sides in contact, hut there is little or no distortion of the concentric 

 zones as a result, and tlie line of contact is a straight line (plate 21, fig- 

 ure 2). 



OOLITES /iV THE YELLOW SHALE 



The yellow shale is distinctly less sandy than either the red or the green 

 shale. The spacing of the oolites in this rock is like that in the red shale 

 (plate 22, figure 1). Under the microscope the ground-mass is seen to 

 be opaque yellow clay, and occasionally there are a few grains of calcite. 



The oolites are of two sizes, as in the red shale. The larger ones aver- 

 age about .62 millimeter in diameter and the smaller a})out .156 milli- 

 meter, ranging from .130 millimeter to .260 millimeter. The larger 

 oolites comprise about 14 per cent of the slide and the smaller more than 

 50 per cent. It should be noted that the large oolites are or approximately 

 the same size as are those in the red shale (see plate 21, figure 1, and 

 plate 22, figure 1). 



The oolites, especially the large ones, are concentrically banded, the 

 concentric rings being due to variations in the diaphaneity of the material. 

 There is usually an opaque central sphere in the large oolites which is of 

 nearly the same diameter, .09 millimeter, in all. This o[)aque zone, with 

 its uniform diameter, occurs in the small oolites also wlicii their diameter 

 exceeds that of the opaque zone. The next prominent concentric sphere 

 is .368 millimeter in diameter and occurs in all the large oolites, Avhether 

 or not the above iioted central opaque zone is present. Immediately out- 

 side of this sphere are four concentric spheres having a total width of 



