210 DYNAMICAL GEOLOGY. 



which takes 10 to 14 days to settle in pure water, settles in 14 to 18 hours 

 in a solution of common salt (W. H. Sidell, 1838). A tine clayey precipitate 

 goes down in a solution of the strength of sea water in .30 minutes, which in 

 pure water would take as many days (W. H. Brewer, 1883). 



Since the chief part of oceanic work requires the presence of rock-mate- 

 rial, depth of water is a condition of prime importance. It is only witliin 

 shallow depths that the waters come extensively into working contact with 

 rocks ; only in the shallow belt where Avater and rocks are together along the 

 emerging line that the greatest amount of force is generated for work. Being 

 at a depth of 500 feet in the ocean is not as complete removal from oceanic 

 forces as being 500 feet above it, but the geological results produced at thi& 

 and greater depths are relatively small. As explained beyond, there are 

 wide differences between the work of the upper 10 fathoms along shores, and 

 that of the depths from 10 to 100 fathoms ; of greater depths along the sides 

 of the oceanic basin when reached by marine currents ; and of depths from 

 100 fathoms to abyssal depths, remote essentially from all currents. 



It is therefore obvious that the era in geological history when the ocean 

 carried on the greatest amount of rock-making was that of general con- 

 tinental submergence at shallow depths, with a scattering of emerged rocky 

 ridges or areas. This was the condition of the earth throuarh the Paleozoic 

 eras ; and, to a large extent, through Mesozoic time. The condition was in 

 striking contrast with the later and present state, in which the continents 

 have only a narrow margin of shallow water. This fact should be kept in 

 mind when comparing ancient geological events with modern. The time of 

 the greatest amount of ocean work was that of the least amount of river 

 work. 



Characteristics of the Working Agencies. 



1. The Tidal Wave. 



The tidal wave moves as a force wave, and has a mean height, along 

 coasts where least influenced by the land, of less than a foot. The height on 

 the projecting capes of continents is 1 to 2 feet, but along intervening 

 coasts commonly from 4 to 12 feet, and in bays and straits, 15 to 18 feet 

 or more. Along the east coast of North America, southern Florida, Cape 

 Hatteras, and Nantucket are the dividing points between a " Southern," 

 "Middle," and "Eastern" Bay (Bache). The height is 1 to 11 feet at 

 southern Florida, 2 at Cape Hatteras, and 1 at southeastern Nantucket ; 

 but in the Southern Bay at Savannah it is 7 feet ; in the Middle Bay, at 

 New York, it is 5 feet ; in the Eastern, at Boston, 10 feet. 



Up deep bays, when the tide enters between strongly converging coast 

 lines, the wave increases much in height. At the Bay of Fuiidy, an unusu- 

 ally long wave enters and reaches a height of 40 feet, and even 60 to 70 feet 

 at the highest tides ; the advancing wave is like a moving water-fall of 

 majestic extent, but without foam. At the entrance to the Bristol Channel,. 



