396 The Americaii Geologist. June. 1900 
direction contrary to the direction of gravitation. Occasion- 
ally the gypsum assumes a botryoidal form, but is commonly 
found as needles or aggregated in loose masses of fibrous 
crystals. The gypsum crystals occur only along cracks, and 
are built up by increase from the base, while the calcium-car- 
bonate stalactites are always built up by additions to their sur- 
face or terminations. It is dilBcult to account for the large 
amount of sulphur needed by assuming its origin in organic 
bodies, such as plants and the forests which are now of abund- 
ant growth in the region and have been so for geologic ages. 
The origin of the carbon dioxide necessary for the great work 
of solution which has been accomplished is likewise found in 
the decaying vegetation. The origin of the sulphuric acid re- 
quired to produce these enormous quantities of gypsum crys- 
tals, which have fallen so abundantly as to fill up certain ave- 
nues, is still problematic. Only one other mineral is found, — 
flocculent crystals of magnesium sulphate, pendant from the 
ceiling of two or three small rooms. There are no calcite or 
quartz crystals. The paper was illustrated by lantern slides 
showing the peculiarities of the stalacitite and gypsum for- 
mations. 
Professor Kemp, in discussion, suggested that the small 
percentage of sulphur present in the limestones themselves 
might, after the solution of the latter, aggregate sufficient sul- 
phur to afford gypsum along the crevices. Doctor Julien and 
professor Stevenson each cited cases in the Carribbean and 
Bermuda islands where the amount of vegetation now or 
formerly growing on the surface was insufficient to accom- 
plish the solution required for the great caves which exist in 
the coral limestones, both of Tertiary and recent growth in the 
islands. 
A paper by doctor E. O. Hovey, on the "Scenery of the 
Harney Peak District in the Black hills. South Dakota," (illus- 
trated with lantern slides) was read by professor Kemp, owing 
to the former's absence. After a brief resume of the geology 
of the Black hills district, a series of views was shown illus- 
trative of the extraordinary erosion forms of the schists and 
pegmatytes of the Harney Peak district. The photographs 
also showed the tin mines of the Black hills, in which spodu- 
mene crystals of large size have recently been obtained as a 
valuable source of lithium, as a commercial product. One 
spoduraene crystal here obtained was thirty feet long. The 
granite veins have also been described by Van Hise. 
Discussion followed on occurrences of extraordinary 
large crystals of other minerals. 
