THE FORMATION OF GEODES WITH REMARKS ON THE 

 SILICIFICATION OF FOSSILS. 



By Ray S. Bassler, 

 Curator, Division of Invertebrate Paleontology, U. s. x<iii<>ii<tl Museum. 



Spherical, or irregular spheroidal, concretion-like masses, hollow 

 and lined with crystals pointing inwardly, are known to geologists 

 as geodes. Externally the similarity of these objects to concretions 

 is so great that breaking is often required to determine their true 

 nature. In matter of origin, there is little, if any. relation between 

 the two. Geodic cavities are common in all kinds of rocks. Amyg- 

 daloidal cavities in igneous rocks are sometimes lined with crystals, 

 thus giving rise to rounded masses coming within the above defini- 

 tion. Openings in veins of ore likewise may be lined with crystalline 

 matter, and the term geode is sometimes applied even though the 

 resulting object is far from spherical in shape. More often, however, 

 these structures are found in sedimentary rocks, limestones and shales 

 being particularly favored, but the general method of formation 

 ascribed to them, namely that of the deposition of crystals from some 

 mineral solution upon the walls of a cavity, is the same in all in- 

 stances. Several limestone and shale formations have been so pro- 

 lific of geodes that they are well known in geologic literature under 

 such names as geodiferous limestone or the geode beds. Thus the 

 geodes in the Niagara (Lockport) limestone, particularly at Lock- 

 port and Rochester, Xew York, early gave the name geodiferous 

 limestone to the formation. These geodes are simply crystal-lined 

 cavities, often of considerable size and number, scattered through 

 the rock. The cavities owe their existence in many cases to the re- 

 moval by solution of fossils; again they are simply fractures in the 

 strata which become enlarged by the expanding force of crystalliza- 

 tion. The material in these particular geodes is usually dolomite or 

 gypsum, and the individual crystals are often large enough to make 

 handsome mineralogical specimens. Some of the more striking- 

 materials found in these Niagara geodes are masses of silvery selenite, 

 crystalline anhydrite, and crystals of amber-colored calcite and of 

 pinkish dolomite. Similar occurrences of geodes in other limestones 



Proceedings U. S. National Museum, Vol. XXXV— No. 1637. 



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