The supposition therefore that this grade is composed in part of kaolinitic grains cemented together by some alkaline salts, finds sup port in any plausible assumption that may be made.
(0.005-0.02 and (0.001-0.005) groups: If the kaolin grains in these groups were in their natural condition, i. e., flat crystals, they should, theoretically, be visible through the microscope. This evi dently was not the case. Beyer and Williams' say : "While it is next to impossibleto make out much concerning the crystalline character of the minerals, it is also difficult, because of their minute size in most secondary clays, to say anything regarding their shape."—In other words, the shape of the grains is irregular and non-conformable one with another. If in these grades there is as much kaolin as is shown in Table XVIII, its grains must be, to a very great extent, in bundles. If feldspar or mica is present in such amount and size of grain, as the cal culated data suggest, their grains ought to be detectable with the aid of the microscope. Such, however, is not the case. The supposition, therefore, that all the akalies of these two grades are there as constitu-. ent parts of feldspar and mica is certainly untenable.
(0.000-0.0001) grade: The molecular composition of this group is certainly very instructive. That in such heterogeneous mixtures as shales and river clays the finest particles are found to be composed in the main of kaolinitic grains is certainly astonishing. If the bases present, as shown in Table XIX, page 181, are considered as being present as soluble salts that were either originally present in the clays or in part introduced during the process of analysis, (a most plausible assumption) then there would remain but one conclusion, that is, that the finest insoluble grains are almost entirely kaolinitic in composition.
Taking data given by Grout, it was calculated that if all the soluble salts originally in the clays were in the finest group, they would amount to 2.7 per cent of the weight of that group. The 2.7 per cent, together with the soluble salt introduced during the process of analysis from glassware, water, atmospheric dust, etc., would account for nearly all of the alkali in the finest grade. It is not mere assumption therefore, that the finest particles in clay, contrary to Grout's statement, are the purest kaolin grains.
In the course of the research on paving brick clays by the survey there was much speculation as to the number of these submicroscopic kaolin grains in the various shales. This was readily ascertained as follows : By dividing the percentage amount of the group (0.001 to 0) by 100, and considering that as being a part of 1 milligram of the sample, (for the size of the particles is in millimeters) then dividing this amount by the specific gravity of the clays, a figure is obtained that represents the sum or total volume in cubic millimeters of the particles comprising the group. Considering 0.0005 as the mean diameter of the particles,
6 by the formula the volume of each particle is found to be cubic millimeters. Then for each day, by dividing the total volume of the particles, by the volume of one particle, the number of grains per of the sample will be obtained. By multiplying the num ber of grains in one milligram by 1,000 there would be obtained the number of grains in 1 gram; or by multiplying by 352,740 there would be obtained the number of grains of this size in 1 oz. of the whole sample. In this way Table XXI was calculated.
These figures, although beyond the limits of perception of the human mind; are not larger than the figures representing the countless germs that bacteriologists claim can exist in a single drop of a fluid. Startling as this data appears to be, it cannot be other than true if the analytical results of the mechanical separation are correct.
If these minute particles were not kaolin grains, would they add to the real plasticity of the clay? Potter's flint (dry ground) is finer grained than most clays, and particularly more so than the shales, yet it does not exhibit the faintest sign of plasticity. Orton' found that glass particles which were so fine that they remained in suspension for hours without settling, when collected exhibited no plasticity. Wheeler' found that while quartz crystals ground to 200 mesh, seemed to be ap preciably plastic, on drying the coherence was so slight that it required the gentlest handling to prevent the molded sample from falling to pieces. Fine quartz dust and impalpable geyserite or finely precipi tated opal, dried to a very tender mass. The same was true of tripoli. Wheeler' found that some plasticity could be developed in powdered slate, prophylite, talc, gypsum, halloysite, etc., but that the plasticity developed was only apparent plasticity, except perhaps in the case of slate. The powdered gypsum when molded and dried formed a rela tively hard mass, but this hardness would be expected on account of the solubility of gypsum in water. The plasticity of the slate, which is a dehydrated shale has caused considerable surprise, and has strengthened the fineness of grain theory of plasticity. That powdered slate should develop plasticity need not be so great a source of wonder, for in the course of the Survey work a shale, after having been held at heat rang ing from 500° to 800° C. for 17 hours, slaked down in water to a red plastic mass in the same manner as the unburned shale at the bank. True, the plasticity of this partially burned shale was not equal to the plasticity shown by the clay before dehydration, but its plasticity was considerably more than that of some of the harder shales before being burned. Fineness of grain in itself then does not, seem to be the cause of plasticity. It may said, however, to be a required condition in the operation of the real cause.