Barbour and Torrey — Microscopic Structure of Oolite. 247 



The kind shown in fig. 1 is the familiar concentric type, the 

 other, shown in fig. 2, illustrates one with a sort of brecciated 

 spherule, each being composed of a mosaic of exceedingly 

 small fragments cemented together about a center. Though 

 differing so widely under the microscope, these two are essen- 

 tially alike, showing only slight external and chemical differ- 

 ences. Other aspects of these and other specimens will be 

 fully considered at another time. 



While examining these specimens, a siliceous oolite was 

 received from eastern Pennsylvania, which merits description. 

 Popularly the name oolite presupj)oses a calcareous rock, yet 

 we have at hand several characteristic oolites which are not 

 calcareous ; these include an iron-oxide form, a strictly siliceous 

 kind, a silica-lime, and a lime-silica form. The last two seem to 

 be transitional forms between the true lime or calcareous oolite 

 on the one hand and the siliceous on the other. In the siliceous 

 oolite the roe and matrix are so intimately united, both being 

 nearly pure silica, that the conchoidal fractures, in the denser 

 sorts, pass alike through spheres and cement, as if it were a 

 homogeneous quartzite. In the more friable sorts the concre- 

 tions pull away intact from the matrix, and present to the eye 

 the familiar fish-roe appearance of oolite proper, for which it 

 could easily be mistaken by a careless observer. A second 

 glance, however, reveals certain superficial differences, in hard- 

 ness, color, etc. In the siliceous oolite, the concretions — which 

 are noticeably uniform in size and spherical in form — are con- 

 siderably darker than the surrounding matrix, being almost 

 black in some specimens. On the other hand, those in the 

 true oolite are generally of a much lighter color than the cal- 

 careous cement. A fractured surface of the siliceous variety 

 exposes the component spherules in section, showing their 

 concretionary structure — their concentric coats of alternately 

 lighter and darker color, deposited around real or imaginary 

 centers. In many, organic remains are the nuclei ; in others, 

 crystals or fragments of inorganic matter. 



A polished surface of this oolite slightly etched with hydro- 

 fluoric acid shows to perfection these agatized bands ; and a 

 microscopic section brings them out to still greater advantage. 

 In such a section, under a low power, it is interesting to notice 

 the numerous rings of various widths and colors arranged 

 around their respective centers, and to further observe how 

 this concentric arrangement continues on into the matrix of 

 the interstices not unlike the coast lines on a map. Often the 

 interspaces are filled with aggregations of minute spherules 

 (not visible to the eye), which also show concentric rings (see 

 figs. 5 and 6). In several instances, lines of faults run through 



