GEOLOGY IN NATIONAL MUSEUM MERRILL. 265 



representing the solidified sand of a very ancient sea beach. Be- 

 sides preserving the original ripple marks, it is crisscrossed by 

 tracks not unlike those of an automobile tire but in reality repre- 

 senting the impressions left upon the sea beach by a large wormlike 

 animal. Following this fossil sea beach is a base containing slabs 

 of shale, sandstone, and coralline limestone, each crowded with fos- 

 sil remains and representing types of near-shore sedimentation. 

 Nearby is a mount of conglomerate, a type of rock in which fossils 

 are less often found. Each of these tells its own story. One speci- 

 men, the " edgewise " conglomerate, is a fragment of a formation 

 H,000 feet thick, extending throughout the Appalachian area and 

 made up of slivers of limestone standing on edge. Another has a 

 different history, as it is composed of rounded quartz boulders. A 

 third contains only angular boulders of limestone forming a 

 " breccia," while still another is a consolidated mass of scratched 

 glacial pebbles. 



Near at hand is a large slab of Devonian limestone composed 

 entirely of the stems and calices of a species of gigantic fossil sea 

 lity. This slab formed part of a layer outcropping along the bluffs 

 of the Mississippi River south of St. Louis, Mo. Passing other 

 exhibits in this series, attention may be directed to a limestone slab 

 of considerable size showing a stratigraphic unconformity, or ex- 

 pressed otherwise, affording an actual illustration of the dividing 

 line as registered in the rocks, between two great periods of the 

 earth's history. Nearby are large masses of limestone showing the 

 ancient plant and animal remains to which they owe their origin. 

 Another large rock fragment illustrates the formation of the well- 

 known fertilizer, calcium phosphate, from small phosphatic shells. 



The differences between the faunas or assemblages of animal life in 

 the different geological periods is shown by two exhibits. 



The first, from the Frank Springer collection, is a fragment of a 

 Cretaceous sea bottom exhibiting a colony of fossil crinoids (Uinta- 

 crinus socialis Grinnell). The slab (pi. 3, fig. 1) is 7 by 8 feet in di- 

 ameter and is the remnant of a lenticular plate, originally upward of 

 50 feet in diameter, composed of the remains of a large colony of these 

 free-floating organisms, which swam vigorously by means of their 

 long arms. They frequently collected in dense swarms with arms and 

 pinnules intertwined, and thus perished, the entangled mass sinking 

 to the bottom, where, as in this case, it was flattened and embedded in 

 the soft mud. With time and pressure the mass became consolidated 

 into a thin plate of limestone, with the crinoids well preserved on 

 the under surface. This, after being lifted out of the quarry, and 

 freed from the fine adherent matrix, is now exposed to view. Nearly 

 all of this colony was destroyed by erosion before its discovery, and 

 the parts recovered were necessarily more or less fractured, but the 



