72 FLORIDA GEOLOGICAL SURVY-I5TH ANNUAL REPORT

aluminous silicates, as feldspar. It is white in color, slightly plastic, has a hardness of 2-2.5 and a specific gravity of 2.2-2.6.
 Crystals of kaolinite are of very rare occurrence in clays but have been described by several writers.2 According to Hickling3 the kaolinite occurs in irregularly hexagonal prisms with rough faces which show strong transverse striations that correspond to the basal cleavage.
 There are several niinerals very closely related to kaolinite. These are all hydrous aluminum silicates, but it is doubtful if all commonly placed in this class are really distinct species. They sometimes occur in crystalline form, but more frequently occur in the amorphous condition. These minerals are halloysite, indianaite, pholerite, rectorite, newtonite, allophane, cimolite, montmorillonite, pyrophyllite, collyrite, and schr6tterite. Some of these minerals can be identified by their optical properties.'
 There are many other minerals not related to kaolinite which are often found in clays and do not decompose readily. They may be briefly treated as follows:
 Quartz-SiO2. This mineral is found in practically every clay, though usually in very fine grains and sometimes in very small quantities. It may also occur in the amorphous form. In residual clays the grains are usually angular while in the sedimentary clays they are somewhat rounded, due to the rolling and tossing about they have received by the water action-. In quantity in clays quartz ranges from less than one per cent in some white sedimentary clays to more than eighty per cent in some other clays. Quartz fuses at 18300 C. (cone 35),2 but in the presence of other minerals which act as a flux it may soften at a lower temperature. In clays it affects the fusibility, shrinkage, plasticity, and bonding strength, depending upon the amount and texture of the quartz present.

 2Dick, M., Mining Magazine, Vol. VIII, p. 15, 1876.
 Reusch, H., Jahrb. f. Min., Vol. II, p. 70, 1887.
 Johnson, S. W., and Blake, J. M., American Journal of Science, II, Vol. XLIII, p. 351. 1867.
 3Hickling, G., China Clay; Its Nature and Origin, Trans. Inst. Min. Eng. (England), Vol. 36, 1908-9.
 'Larsen, E. S., The Microscopic Determination of the Non-opaque Minerals, U. S. Geol. Survey Bull. 679, 1921.
 2Ries, H., Clays, Their Occurrence, Properties and Uses, p. 55, 1908.