Geology of Tampa Bay, Florida. 39 
_This limestone in some places is destitute of organic remains, 
in others it contains a great abundance of them, such as univalves, 
bivalves and echini; where it has been exposed to the disintegra- 
ting influences of the atmosphere, beautiful specimens can be de- 
tached. It has in many places sunk down and left depressions 
called limesinks ; which often have a small area and are several 
feet in depth, appearing more like artificial wells than natural pro- 
ductions. ‘The loose texture and proximity of this rock to the 
surface accounts for the fact, that the sandy soil of the country 
contains a large proportion of lime in its composition and possesses 
a greater fertility than its aspect promises. I have never seen 
any chert in the limestone, but from the frequent occurrence of 
nodules of that mineral on the surface, I should infer that they 
came from it. This rock must be several feet in thickness, for 
extensive caverns and galleries are found in it, worn out by sub- 
terranean streams; there is a small excavation near Fort Brooke, 
running many feet horizontally and nearly high enough for a 
man to stand upright. 
There is another rock that probably dips beneath the lime- 
stone—a dark bluish, silicious rock, of a compact texture, some- 
what vesicular, the vesicles containing minute crystals of quartz. 
This rock can be seen at the rapids of the Hillsboro’, about nine 
miles north of its mouth. I have noticed bowlders of it at seve- 
ral places in the interior, particularly near the forks of the With- 
lacoochee river, containing beautiful impressions of bivalve shells. 
This and the limestone are the only compact rocks I have ever - 
seen in the peninsula. Small bowlders of a gray sandstone occur 
a few miles north of Fort King. 
In the vicinity of the Hillsboro’ river and other streams that 
discharge their waters into Tampa Bay, are several sulphur springs, 
which issue from crevices in the rocks, and supply a copious quan- 
tity of water charged with sulphureted hydrogen gas. ‘There is 
a very large one near the west bank of Hillsboro’ river, about 
eight miles north of Fort Brooke ; it is thirty feet or more in di- 
ameter and twelve feet deep, and issues from a crevice in the un- 
derlying rock, throwing out sufficient water to form a stream 
more than twenty feet wide and eighteen inches deep, which bub- 
bles up with so much force as to keep the surface in a constant 
agitation like the boiling of a large caldron. The waters of this 
spring are so pellucid that minute fragments of shell glisten on 
the bottom like pearls, and large turtles witha great variety of fish 
