According to Morgan, dams were of two kinds, the "stick" and the "solid bank." The former was made by a combination of sticks and poles on the lower side, while the upper was built of sticks and earth. The sticks were laid in the direction of the current with the butts up-stream, and not across. This was probably due to the animal's inability to lay the stick in any other way, the current itself determining the beaver's conduct, though it is possible that experience had taught that this was the best method, for by such arrangement the water was not wholly obstructed and, percolating through the interstices, was less likely to break away the structure. The other form of dam, the solid bank, was merely a modification of the stick dam adapted to a deeper channel. Large quantities of earth and stones were added to this to enable it to withstand the greater force of water, and this seems to indicate some degree of contemplation on the part of the builder; yet the result was natural, as the animal, having placed earth on one form of dam, would go on placing earth on the same form in deeper water as a matter of instinct. But there was one touch in the construction of the solid-bank dam which more than any other appeared to be the result of thought. This was an opening left in the top of the dam, several inches lower than the remainder, and three or four feet long, as a spillway for surplus water. In the stick dam no spillway was provided because the surplus was allowed to flow through the interstices, so that the construction of this feature in the larger, more compact dams seems to have been an example of pure invention to guard against possible disaster. [Illustration: Great Beaver Dam—Grass Lake. 260 feet long. From Morgan's _American Beaver_] [Illustration: Red Canyon—Green River. Where Ashley Went for Beaver in 1825. Photograph by E. O. Beaman, Colo. Riv. Exp.]