WATER AS A MECHANICAL AGENT. 



221 



Cliffs at entrance to Port Jackson, New South Wales, 

 Australia. D., Note-book, '40. 



193. 



This feature of wave-action, and the reality of a level of little or no 



wear above low tide, are well illustrated by facts observed by the author 



in 1840 on the coasts of 



Australia and New Zealand. ■'•^^* 



In rig. 192 (representing 



the south cape at the en- 

 trance to the harbor of Port 



Jackson, New South Wales), 



the horizontal strata mak- 

 ing the base of the cliff, cut 



crosswise by joints, extend 



out in a platform a hundred 



yards wide. The tide rises 



and covers the platform ; and 



the waves, unable to reach 



its rocks to tear them up, 



because of the protection 



thus afforded, xlrive on to 



batter the lower part of the cliff. The strata belong to the Sydney sandstone 



formation. At the Bay of Islands, New Zealand, there is a similar seashore 



platform, as illustrated in Fig. 193, represent- 

 ing an island in the bay called " The Old Hat." 

 The rock here is a rather firm argillaceous 

 sandstone without bedding. By elevation such 

 shore-platforms become sea-border terraces. 



" The Old Hat," New Zealand. D. '40. Another region of such shore-platforms is the Island 



of Anticosti in the Gulf of St. Lawrence. They have 

 a width there of 100 to 150 yards (Verrill). The broad seashore platform of coral islands 

 or atolls has the same origin (page 146). It occurs on both their leeward and windward 

 sides, and varies little in surface from horizontality. Coral-made limestone, like other 

 kinds, is of easy abrasion. 



As here shown, there is a limit to wave-abrasion. Under the circum- 

 stances stated, it does not cut much below low-tide level. Even an atoll stands 

 its ground and grows in size in spite of the waves of a Pacific. Much less 

 can wave action cut valleys into the land. Its province is to batter down 

 cliffs, wear off headlands, and fill up bays. 



The largest blocks that are raised and carried up seashore are those that 

 are forced along by earthquaJi'e ivaves. These waves commence their tearing 

 work at depths that at other times are under the protection of the waters, 

 and the waters, which had retreated from the shore to make the waves, 

 advance to an unwonted height, and make deposits of what they have gath- 

 ered at varying distances inland, according to their gravity, besides devastat- 

 ing the country they cover. But the depth of their action probably does 

 hot exceed 30 feet. A ship afloat is easily moved landward, more easily 



