176 STUDIES FOR STUDENTS 



of the waste mantle and of the weather-broken rocks beneath, 

 firmly held entangled in the meshes of their roots. The fragments 

 may be expected to be smaller than those of the ice raft. Fine 

 waste is absent from the breccia, since it is soon washed out of the 

 interlacing roots of trees when immersed in river or sea. Since 

 the fragments are deriyed from the zone of weathering, decom- 

 posed and etched surfaces and weather-rounded edges may be 

 looked for, especially in fragments made of the more soluble rocks. 



Seaweed breccia. — The buoyant power of seaweeds and the 

 tenacity with which they adhere to rock are well known. They 

 are thus able to transport stones so small that they would readily 

 escape from the roots of floating trees. At the Orme's Head, North 

 Wales, angular fragments of limestone have been found attached 

 to the roots of Laminaria. 1 The stones which seaweed commonly 

 carry are the well-worn shingle of the pebble beach. But angular 

 stones may be transported by them when fragments are broken 

 by the battering of waves from the rocky reefs on which seaweeds 

 grow. 



A pudding breccia occurring in one or two localities near Dublin, 

 Ireland, has been attributed to tree rafts by Jukes 2 and to carriage 

 by seaweed by Ball, 3 although earlier observers had invoked rafts of 

 floating ice. The matrix is highly fossiliferous, encrinital, car- 

 boniferous limestone. The fragments, sharp-edged, sporadic, small, 

 are of granite and metamorphic rock outcropping in the neighbor- 

 hood. The small size of the fragments lends some weight to sea- 

 weed as the transporting agent. 



Desiccation breccia.* — -Surface layers of unconsolidated fine- 

 grained sediments, such as clay or limy mud, when exposed to the 

 air, dry, shrink, and sun-crack. The angular blocks of this mosaic 

 may again be covered with water and imbedded in the sediments 

 which it throws down. The conditions for desiccation breccia 

 are afforded where there are long intervals between periodic 



1 C. E. de Ranee, Quar. Jour. Geol. Soc. London, XLIV (1888), 374. 



2 Jukes, Manual of Geology (1886), p. 298. 



3 V. Ball, Quar. Jour. Geol. Soc. London, XLIV (1888), 371-74. 



4 Desiccation conglomerates is a term proposed by J. E. Hyde, Amer. Jour. Sci., 

 4th ser., XXV, 400 f. 



