in all directions from a centre, whereas not one even of the smaller ones is found to the westward of A, though a very slight force would have made them roll down to the base of that ridge, which is very steep on its western declivity. It is clear, therefore, that the propelling power, whatever it may have been, acted exclusively in a south-easterly direction. Professor Hall and I observed one of the green blocks--24 feet long, poised upon another about 19 feet in length. The largest of all on the west flank of m, or Dupey's Mount, called the Alderman, is above 90 feet in diameter, and nearly 300 feet in circumference. We counted at some points between forty and fifty blocks visible at once, the smallest of them larger than a camel. [Illustration: Figure 51. Dome-Shaped Block] (FIGURE 51. ERRATIC DOME-SHAPED BLOCK OF COMPACT CHLORITIC ROCK (n in map in Figure 50), near the Richmond Meeting-house, Berkshire, Massachusetts, latitude 42 degrees 25 minutes North. Length, 52 feet; width, 40 feet; height above the soil, 15 feet.) The annexed drawing (Figure 51) represents one of the best known of train Number 6, being that marked n on the map (Figure 50). According to our measurement it is 52 feet long by 40 in width, its height above the drift in which it is partially buried being 15 feet. At the distance of several yards occurs a smaller block, 3 or 4 feet in height, 20 feet long, and 14 broad, composed of the same compact chloritic rock, and evidently a detached fragment from the bigger mass, to the lower and angular part of which it would fit on exactly. This erratic n has a regularly rounded top, worn and smoothed like the "roches moutonnees" before mentioned, but no part of the attrition can have occurred since it left its parent rock, the angles of the lower portion being quite sharp and unblunted. [Illustration: Figure 52. Position of Block in Figure 51] (FIGURE 52. SECTION SHOWING THE POSITION OF THE BLOCK IN FIGURE 51. a. The large block in Figure 51 and n in the map in Figure 50. b. Fragment detached from the same. c. Unstratified drift with boulders. d. Silurian limestone in inclined stratification.)