gravels variously graded, whether it lasted for one hour or for 24, or 48 there was always a minute proportion only of material between 1/30th and 1/100th inch (0.59 and 0.149 mm) while the amount between 1/20th and 1/200th inch (1.18 and 0.07 mm) was unimportant. This was the case even in those instances when one and a half grams and more were being reduced from the condition of fine gravel, to that of silt and clay every minute. It is clear that the first effect of impact, must be mainly the production of grains of material between 1/20th and 1/200th inches; yet at any one time, the amount of material of this size in all of the samples, was found to be negligible. The suggestion is clearly that material of this grade was destroyed, almost directly it was formed, by some action which operated with great rapidity. Further experiments were made in order to ascertain the actual rate at which the fine gravel and sand were changed into silt and clay. A sample was first employed, consisting of 4,500 gm. 11/2 to 1 inch, (38.1 to 25.4 mm), and 500 gm. of quartz-sand from Otago, which graded as follows:— mm. mm. gm. Sieve 20–40 1.18–0.42 7.3 1/40–1/50 0.42–0.28 18.5 1/50–1/60 0.28 0.24 37.2 1/60–1/70 0.24–0.19 120.3 1/70–1/80 0.19–0.17 32.5 1/80–1/100 0.17–0.14 178.6 1/100–1/200 0.14–0.07 94.8 Passed 9.8 After this sample had been treated in the Deval Machine for 24 hours, 0.01 gm. of sand only were retained on the 200 inch sieve; while the material between 0.07 and 0.04 mm weighed 10 gm., that between 0.04 and 0.01 weighed 116 gm., and the material finer than 0.01 mm weighed 374 gm. It was clear from this experiment that the grinding action of the tube mill type was so rapid that shorter intervals of time were required for experiment. A sample was therefore employed consisting of the gravel used previously, with 500 gm. of quartz-sand, graded from 1/60th to 1/70th (0.25 to 0.19 mm) and treated for one hour. It then graded as is shown in Table 7 Sample J (2). All of the sand was lowered in grade, and the greater part was reduced to extremely fine dimensions. It was treated for another half-hour, and was then practically all in a finer state than 0.07 mm. During the first hour, the gravel lost 14.6 gm., and in the second half-hour, 9 gm; altogether a loss of 0.52% in 1½ hours. The result shows that gravels of this type, weighing 4,500 gm., will in 1½ hours grind 500 gm. of sand into a fine condition, for all but 26 gm., were finer than 0.07 mm. The grinding action is, therefore, far more rapid than abrasion or the effect of impact. When pebbles, 1 inch to 1½ inches (2.54 to 3.81 em) are used, the grinding action begins with fragments of sand 1/30th inch, or 0.59 mm in diameter, and the grinding action continues until material is reduced to the state of silt and clay.