All Erratic Bowlder. — McCaUie. 47 
stones frequently met with in or near the larger streams of 
East Tennessee and North Georgia, but on close examination 
it Avas found to be entirely different both in structure and min- 
eralogical composition. 
The color of the stone is greenish gray, specked with small, 
irregular dark and light colored spots. In texture it is fine- 
grained, none of the individual minerals being made out ma- 
croscopically. Thin sections show that it consists of quartz 
and feldspar phenocrysts and biotite, in a quartz-feldspar 
ground mass. The feldspar and also the biotite arc usually 
much altered, the former giving rise to scales of kaolin, and 
the latter to chlorite. Whether these alterations antedate the 
burial of the rock in the Coal Measures, or are of subsequent 
datC; is an open question. However, the smooth surface of the 
bov\dder would seem to indicate that the stone was polished by 
water when che minerals were in a comparatively fresh state. 
A specimen of the rock submitted to Miss Florence Bas- 
com, of Bryn Mawr, for examination was identified by her 
as an aporhyolyte, though she suggested that the more general 
term metarhyolyte might also be used. ^liss Bascom was un- 
able to decide definitely from a single slide whether the rock 
cooled as a lava flow, or as a dike, yet no doubt was expressed 
as to its classification. 
Tlie most interesting question which suggests itself in re- 
gard to this erratic bowlder is its source. Its water-worn 
surface would seem to indicate that it had probably been trans- 
ported some distance by water prior to its deposition in the coal 
seam, but even this throws littla or no light on its original 
source, as no rocks of like character are to be found in Tennes- 
see or the adjacent portions of Georgia and Alabama. The 
stratigraphical jjosition of the bowlder fixes the time of its 
deposition in the latter half of the Carboniferous age, and as 
a consequence the lava flow or dike from which the bowlder was 
derived must antedate that period. This would seem to indi- 
cate that prior to the latter half of the Carboniferous age at 
some point not far removed from the Etna coal mines there 
were dikes or surface flows of rhyolyte which have since beeri 
buried beneath subsequent deposits. 
