53 
HALL 65. 
STRUCTURAL AND DYNAMICAL GEOLOGY. 
Case lA. — Dendrites. 
Case IB.— Volcanic products, illustrated by lavas, volcanic 
bombs, etc. 
Case 1C. — Cave products, represented by stalactites and 
stalagmites. (See foot note, p. 47.) 
Cases 2 and 3. — ^Specimens representing varieties of rock 
structure. The specimen labels here show; first, the kind of struc- 
ture; second, the name of the object; third, the locality. 
Among the specimens representing concretionary structure, the 
clay concretions are worthy of note, since they assume curious forms 
which are often mistaken by unscientific observers for fossil tur- 
tles, eggs, or fruits. 
Cases 2C and 3. — Septaria, geodes, specimens illustrating 
nodular, oolitic, pisolitic, and spherulitic — which are really varieties 
of concretionary — structure, cellular, porphyritic, and stratified 
structure. 
The septaria are produced from concretions by drying and sub- 
sequent filling of the cracks with calcareous matter. The resem- 
blance of these to a turtle shell is often very striking, and their 
handsome appearance when cut and polished is shown by several 
specimens. 
Case 4. — Specimens illustrating rock texture ; veins ; the 
effects of erosion by water, sands, and other agencies; ripple-marks, 
which are the preservation in stone of lines once made by ripples 
on a mud-flat ; striation and polishing produced by glacial action; 
faulting; and metamorphism. 
Case 3. — Various large specimens of types already mentioned, 
including a large septarium from Perry, New York; a large slab 
showing mud cracks from Holyoke, Mass. ; two slabs showing 
fluting and polishing due to glacial action, from Kelly Island, 
Ohio. In the middle of the room will be found a large slab show- 
ing ripple marks, and a collection of basaltic columns from the 
Giant’s Causeway and the Rhine Valley. The grouping of the 
latter is intended to illustrate the stair-like arrangement which is 
usually seen in basaltic cliffs that have been exposed to erosion. 
Further description and explanation of any of the types of struc- 
ture represented may be obtained from Geikie’s Text Book of 
Geology, Dana’s Manual of Geology, or Le Conte’s Elements of 
Geology, to be found in the Museum Library. 
