Figure 3. Downhole variations in the first-derivative value at 565 nm from the shore-based Perkin-Elmer spectral data set. First-derivative peak height at this wavelength is primarily a function of hematite content (Deaton and Balsam, 1991) and, therefore, can be used as a proxy for the presence of hematite. Note that high values indicative of hematite are found only near the very top of the holes.