54 G. F. BECKER — FINITE STRAIN IN ROCKS. 



this time the pebble will tend to swing across the current so as to present 

 its greatest area to the pressure. As soon as the resistance duo to its 

 inertia is overcome, the pebble will sink through the water as if the fluid 

 were at rest till its edge touches the bottom, and it will then lip down 

 stream till it meets support. In rapid steams irregularities in the bottom 

 cause local upward currents, which project pebbles into the main current 

 much as if they had been dropped into it. These pebbles sink to the 

 bottom again where the movement of the water is more uniform. Many 

 pebbles thus deposited will, with few exceptions, be inclined down stream 

 and will rest against one another, like overlapping tiles. 



This relation explains the fact that both in modern streams and in the 

 ancient river channels containing the auriferous gravels, many of which 

 have been tilted since their deposition, the pebbles, as miners say. 

 "shingle up stream," or, as zoologists would express it, "imbricate" 

 toward the source. Elongated, rOd-like pebbles are usually found lying 

 across the channel. The indication afforded by this behavior of pebbles 

 seems entirely trustworthy so far as the local current is concerned. In 

 applying it, however, it must be remembered that powerful streams arc 

 often accompanied near shore or close to obstructions by local " hack 

 currents," in which the pebbles would be arranged in a direction opposite 

 to that of the main stream. 



If a flat pebble or a mica scale is allowed to subside in relatively quiet 

 water, the fluid may be considered as exerting a pressure on the lower 

 side against a resistance due to the action of gravity on the stone. The 

 disc will then tend to assume a horizontal position. It is for this reason 

 that allothigenetic mica scales in sandstones or other rocks usually follow 

 the direction of the bedding. In massive sandstones this is an assistance 

 in determining the true stratification. 



A very familiar illustration of the action of the strain ellipsoid moving 

 against resistance is afforded by a bubble of gas rising through still water. 

 The spherical bubble is compressed to an ellipsoid, which might be re- 

 placed by a rigid mass of the same density, and it rises with its equator 

 in an almost perfectly horizontal plane. 



On beaches pebbles are sometimes imbricated for a few feet in one or 

 another direction and sometimes lie nearly flat. The constant reversal 

 of the currents due to breaking and retreating waves prevents any exten- 

 sive methodical arrangement, and this fact is of assistance in discriminat- 

 ing marine gravels from river deposits. 



There are also instances of the almost self-evident fact that a rod-like 

 mass moving under the influence of traction, like a vessel under tow, will 

 move end on. In glassy rocks, such as many rhyolites and andesites? 

 the mass often shows a handed structure, marked by the presence of 



