no.1637. ON THE FORMATION OF GEODES— BA8SLER. 135 



many fine cystids and crinoids of the Curdsville formation in the 

 central Kentucky Ordovician would never be known were it not for 

 this replacement of these fossils by silica, nor would the hundreds of 

 species of exquisitely preserved bryozoa, ostracods, corals, and other 

 fossils from the Devonian strata at the celebrated Falls of the Ohio 

 locality be available for study and comparison with specimens from 

 less favorable localities. This phase of silieification has been ex- 

 plained by most writers as due to a replacement of the original cal- 

 careous material of the fossil by silica at the time of deposition. 

 Under this view, the fossil shell, for example, is replaced by silica 

 either on the ocean bottom at the time of deposition or shortly there- 

 after. In the case of a limestone then, the fossil would be simply a 

 siliceous shell embedded in a calcareous groundmass. When in the 

 course of weathering the limestone is dissolved away, the insoluble 

 siliceous shells would then be left behind. 



A few observers have noted that the limestone yielding such 

 siliceous pseudomorphs upon weathering contains, in the unweathered 

 state, only calcareous fossils. They have also observed that, as the 

 weathering of the fossiliferous limestone proceeds, the fossils are re- 

 placed by silica and consequently are not carried away in solution as 

 in the case of the matrix itself. In support of this view, the surface 

 of a fossiliferous limestone layer is often seen sprinkled with fossils 

 of which the portion still embedded in the limestone is calcareous and 

 the exposed parts are siliceous. With this view the time of the siliei- 

 fication is necessarily assumed as very recent, in fact, as going on at 

 the present time. The writer has had many opportunities to study 

 and collect in areas where such silieification has occurred, and all of 

 the facts observed have been in support of the latter idea. The object 

 of this paper is not, however, to discuss this particular form of siliei- 

 fication, and the preceding remarks upon this recent replacement by 

 silica are introduced only on account of their bearing upon the prob- 

 lem of the formation of siliceous geodes. 



The best known siliceous geodes come from the Keokuk geode bed, 

 which consists of blue shale and thin limestone 30 to 35 feet thick, 

 particularly well shown at Keokuk, Iowa, and Warsaw. Illinois. The 

 name of Amos II. Worthen will always be associated with these 

 geodes. since years ago he distributed many barrels full of good speci- 

 mens to the leading scientific men and institutions of this country and 

 Europe. Mr. Worthen's localities are, therefore, best known to the 

 scientific world, and the Keokuk geode bed is even better known than 

 the geodiferous Niagara limestone of New York. The Keokuk geodes 

 vary in size from that of a pea. or even less, up to rounded masses 2 

 feet in diameter. Quartz is the predominating crystalline material. 

 but agate and crystals of calcite and dolomite are often accessory con- 

 stituents. Such metallic minerals as millerite, in the form of hair- 

 like needles or tufts of needles, sphalerite, and pyrite, are likewise 



