140 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



not in regular cavities where the water can circulate freely, but along 

 fracture lines in which capillary attraction is responsible for the 

 penetration of the mineral bearing solutions. Complete series, in 

 which these fractures are occupied by only a thread-like trace of 

 silica up to the normal geode, showing no trace of crinoidal matter, 

 have been seen, but the specimens illustrated on Plates XX and XXT 

 were deemed sufficient for the purpose. Filling of a longitudinal 

 and transverse fracture is shown in fig. 7 of Plate XX; fig. 8 illus- 

 trates three of the five silica-filled fractures of another column. In 

 fig. i) the mineral deposit in the five main fractures has exerted such 

 a pressure that the intermediate crinoidal material has been broken 

 into small fragments. Each of these minor fractures would have 

 developed stringers of silica had the deposition continued. Such 

 minute breaks are illustrated in figs. G and 13, the latter showing the 

 siliceous deposition in a specimen fractured like the original of fig. 6. 

 In fig. 10 the regular arrangement of the silica along five prominent 

 lines is obscured by replacement of side " veins," the original speci- 

 men being, apparently, similar to that shown in fig. 3. Figs. 11 and 

 12 illustrate specimens showing deposition along oblique as well as 

 longitudinal fracture lines. In fig. 14 the mineral matter has been 

 deposited most abundantly at the intersections of two sets of frac- 

 tures, thus producing a botryoidal appearance in the resulting geodes. 

 This botryoidal effect is more pronounced in figs. 3, 8, 9, 10, and 11 

 on Plate XXI, but the same origin is evident from the adhering cri- 

 noidal remains. Such specimens are usually hollow and lined with 

 drusy quartz (figs. 5 and 7). The crust is often so thin that the 

 geode itself may be crushed (fig. 12). In the final stage all trace 

 of the crinoidal material is lost, although the original five fractures 

 often determine the shape and indicate the origin of the geode. Occa- 

 sionally the fragments of crinoidal matter between the ridges of 

 silica are removed by solution and leave fluted masses of silica as in 

 fig. 6. That the geodes of this particular geological horizon are not 

 always confined to fossils for their growth, is evidenced by the frag- 

 ment of crinoidal limestone figured on Plate XXI, fig. 11. Evidently 

 two parallel fractures and a third at right angles have penetrated 

 this specimen and the deposition of silica has followed. Except the 

 last, all of the foregoing illustrations have been based on crinoid col- 

 umns. In the case of the calyx of a crinoid, little or no fracturing 

 is necessary to give rise to the deposition of silica, since the plate 

 sutures furnish an excellent place for crystalline growth. This par- 

 ticular phase is sufficiently illustrated on Plates XIX and XXIV to re- 

 quire no further mention. Nor does it seem necessary to describe the 

 various stages in geode growth starting from a brachiopod as Plates 

 XXII and XXIII are deemed explanatory enough. As the arrange- 

 ment of the fractures in crushed brachiopods is usually indefinite, the 

 geodes resulting from this class are traced with more difficulty. It is, 



