Three separate sources of phosphate were actively mined in Florida during the 1890s: hard rock, river pebble, and land pebble. Hard rock mining, such as that shown in the photograph, began after teeth, tusks, and large bones rich in phosphate were discovered at Dunnellon in 1889. Hard rock mining surpassed the other phosphate sources during most years of the 1890s, but after 1906 output declined. River pebble phosphate, occurring particularly in shallow sand bars in the Peace River, was mined until 1908. Boats equipped with steam dredges were employed in mining the bars. Land pebble became the leading phosphate source after 1905 owing to its relatively uniform occurrence in Polk and Hillsborough counties.
