80 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 13, 



line crag there are seldom continuous beds of great extent, but a 

 simple arrangement of diagonal laminae. Portions of the lower green- 

 sand present us with instances where the com^ponent strata are carried 

 on for great distances, with their upper and lower planes strictly 

 parallel, and vriih. cross structure throughout in the same dhection : 

 in these cases the angle of lammse is uniformly higher than in the 

 more confused and shorter bedding. The angle of rest for drifted 

 sand will be liigh in proportion to the stillness of the water into 

 which the sand-bed is extenduig itself ; so that in these seyeral well- 

 known characters of deposits we seem to have a guide as to the con- 

 dition of depth and directions of moving power over these old sea-beds. 

 In the torrential structure of river deposits the dhection of the 

 bedding is with the stream. In marine beds we constantly meet with 

 it, as in the pleistocene diift and crag deposits, setting in opposite 

 directions in consecutive beds, as contradistinguished from those 

 setting constantly in a given direction. In the first case the mate- 

 rials are mostly mixed and coarse — laminated sands, between hori- 

 zontal gravel beds ; m the latter the materials are finer, and indicate 

 the undisturbed process of constant outward accumulation. In the 

 confased stratification alluded to, and in the thin layer of sand which 

 is so constantly intei-posed between masses with opposite diagonal 

 bedding, we have an arrangement which may be due to tidal influence. 

 If we take a portion of the east end of the Channel, and where the 

 tidal movement of the water is well known, we find that the flood at 

 the syzygies runs 4^ 40', at the ebb 6^ 50' ; that the minimum 

 velocity, which depends on the height of the tide, is at most half of 

 the maximum velocity, or that with high tides the flood is more rapid 

 than the ebb. Making allowance for these different forces and direc- 

 tions, the confused arrangement to be observed in these deposits be- 

 comes intelhgible. 



In the deeper sea sand beds, which travel outwards by additions in 

 front, the formation of a single stratum may be continued until the 

 effect of its accumulation is such on the body of water entering such 

 sea as to admit of a new bed being formed above it ; so that the 

 interval of time between one stratum and another in this zone may be 

 very great. The dimensions of strata of this kiud are greater than 

 those presenting other arrangements. Diagonal beds of great thick- 

 ness may be noticed hi the crag deposits, and I have seen them up- 

 wards of twenty feet thick ui the lower greensand, a thickness which 

 would correspond with a sudden increase in soundings of ft'om three 

 to four fathoms. 



A Tery common form of arrangement is where the lines of deposi- 

 tion are parallel with those of bedding ; hi this case we seem to have 

 no guide as to zone of depth. We meet with it in the so-called raised 

 beaches, which are marginal sea-beds, ranging down to 10-15 fathoms. 

 The stratification of the beds at Braunton is most distinct and re- 

 gular. This mass is the shallow water accumulation of a sheltered 

 bay ; but w^e may trace the same arrangement in beds which were 

 evidently deep and wide sea deposits, such as those of the white 

 chalk, orwhere like masses have been altered into crystalhne limestones. 



