meters, and consisting of quartz (Q), mica (M), feldspar (F) and lignite (L), the cloudiness of the feldspar being due to partial kaolinization. Fig. 33 shows the grains of fine and medium sand, and Fig. 34 the clay particles, the former being enlarged 115 diameters, the latter 362 diameters.

An unburned clay owes its color commonly to some iron compound or carbonaceous matter, a clay free from either of these being white. Carbonaceous matter will color a clay gray or black, depending on the quantity present, 3 to 4 per cent. being usually sufficient to produce a deep black. A sandy clay will, however,

be more intensely colored by the same quantity than one with many clay particles.

Iron oxide colors a clay yellow, brown, or red, depending on the form of oxide present. The greenish color of many of the Clay Marls is due to the presence of the mineral glauconite.1 The iron coloration is, however, often concealed by the black

Showing grains of sand in a Clay Marl I, enlarged 115 diameters. M, mica; Q, quartz; F, feldspar; L. lignite.

coloration due to carbonaceous matter. It is often more or less difficult to make even an approximate estimate of the iron content in a clay from its color. Thus, for example, two clays from the Perth Amboy district which were of nearly the same

color, had respectively 3.12 per cent. and 12.46 per cent. of ferric oxide.

The color of a green or raw clay is not always an indication of the color it will be when burned. Red clays usually burn red; deep yellow clays may burn buff; chocolate ones commonly burn red or reddish brown; white clays burn white or yellowish white, and

Grains of fine and medium sand from a sample of Clay Marl I, enlarged 115 diameters.

gray or black ones may burn red, buff or white. Calcareous clays are often either red, yellow or gray and may burn red at first, but turn yellow or buff as vitrification is approached.

SLAKING.

When a lump of raw clay is thrown into water it fall to pieces or slakes, but the rapidity with which this takes place varies

greatly in different clays. Open, porous, sandy ones fall rapidly to a powdery mass; others may spall or chip off slowly when immersed, while still others either do not slake at all, or only after long soaking. The slaking property is one of some practical importance, as easily slaking clays temper more readily, or, if

the material is to be washed, it disintegrates more rapidly in the log washer.

The slaking qualities of the clays from the different formations are referred to in Chapter XVIII, where the different groups are discussed collectively.

SPECIFIC GRAVITY.

The specific gravity of a clay stands in more or less close relation to the density of its mineral particles, and affects its weight per cubic foot, but most clays do not vary much in their specific gravity, ranging commonly from about 1.80 to 2.60. That is they weigh 1.80 to 2.60 times as much as an equal amount of water.

The specific gravity of a number of samples from New Jersey was determined and the figures are given below. It will be noticed that there is not as much variation as one might expect.

If we compare these figures with the figures obtained by Prof. Cook in 1878, we find that his were in most cases much lower, but the reason for this is due to the fact that the determinations for this report were made on the clay powder with a pycnometer, while those in the 1878 report were on lumps of the clay coated