1882.] 



On the Formation of Ripplemark. 



7 



crossed by 2-inch ripples from the north-east. The explanation of these 

 cross ripples was clear. A portion of the sands was raised to such an 

 extent that on the tide retiring an island was formed for a short time. 

 The 6-inch ripples owed their origin to the swell from the south-east, 

 whilst all the sands were covered, whereas the 2 -inch ripples owed 

 theirs to the water running in and ont at the back of the emerged 

 sandbank at right angles to the direction of the main swell. 



On the 23rd January I went over the same ground again, with the 

 following results. Under Salyarde Terrace I again found the ripples 

 coming from the south-west. Under the Belgrave Road their direc- 

 tion was due south, well developed on flats, but obliterated on slopes 

 towards the sea. In the " submerged forest " clay was a round pool, 

 15 feet in diameter, with its bottom covered with 5-inch ripples from 

 the south-by-east (S.b.E.), crossed by 1^-inch ripples from east-by- 

 north (E.b.N\) This was owing to the southern side of the pool 

 drying before the eastern. On a small beach between the rocks, at the 

 western ends of the sands, under the Great Western Hotel, the ripples, 

 of different sizes but averaging about 3 inches, came as nearly as pos- 

 sible from the south-east. Under the Oorbons Head I again found 

 the large ripples, the largest being 14 inches between ridges and 

 2 inches high ; they were composed of sand, coarser than at Torre 

 Abbey, and broken shells. At the east end of Livermead Sands there 

 was again an extensive low bank, with pools on the landward side. 

 On the bank were 3-inch ripples from the S.K., gradually obliterated 

 towards low-water mark, where the sand was quite smooth. One of 

 the back pools was covered with perfect ripples, varying in size from 



2 inches (by estimation, as they were inaccessible) to 17 inches by 

 measurement. Direction of all, south-east. The landward slope of 

 the sand- bank was covered with ripples from the south-east, crossed at 

 different places by others from the north-eastward and eastward. At 

 the west end of the Livermead Sands were some large, but not per- 

 fectly preserved, ripples, 22 inches long between ridges and over 



3 inches high. 



These observations prove that ripplemarks are independent of the 

 direct action of wind, for on two separate occasions the Torre Abbey 

 and Livermead Sands could furnish at the same time ripples coming 

 from all points, from south-west to north-east (on the eastern side). 

 They also show what a complicated problem is that of the size of 

 ripplemarks, and how little the geologist can gather from mere size, 

 for on the same beach were ripples ranging from 1-J inches to 22 inches, 

 and in one small pool almost every size was represented between 

 2 inches and 17 inches. 



Having shown that the conditions requisite for the formation of 

 ripplemarks are alternating currents on a mobile bottom, I will pro- 

 ceed to show that there is good evidence that alternating currents and 



