144 



THE GEOLOGIST. 



of ripple-drift. The layers parallel to a b may be called the " ripple- 

 drift-bands whilst the smaller layers of which these are composed 

 may be distinguished by the term "stratula." The thickness of the 

 beds, the length of the bands, and the number of the stratula, may be 

 indefinite, whilst their other peculiarities are definitely related to the 

 circumstances under which each bed was formed. 



The "ripple-drift-bands" are the lower portions of the laminated 

 ripples of the upper surface, which were not washed up, but covered 

 over by the next-following ripples advancing over them. They are, 

 therefore, made up of oblique bedded stratula, and in this respect 

 resemble a single bed of drift-bedding ; but, from the nature of the 

 case, they are inclined to the true plane of horizontal stratification at 

 the angle ah c ; whereas a single bed of drift-bedding is parallel to it. 

 They also usually differ much in size. I have seen beds of diift- 

 bedding twenty-five feet in thickness, and beds several feet thick are 

 very common, whereas the thickness of an inch or two is considerable 

 for the " ripple-drift-bands." Still, in some cases, the size is quite 

 similar, and then it requires care to distinguish a band of " ripple- 

 drift " from a bed of drift-bedding," notwithstanding that they differ 

 so essentially in their origin and relationships. 



If anyone will reflect on the manner in which the " ripple- drift- 

 bands" are formed, he will perceive at once that their thickness 

 indicates the excess of material deposited on the sheltered side of the 

 ripples over that washed up again from the exposed side, during the 

 time required for each ripple to advance a distance equal to its own 

 length, which time we may conveniently call its " period." The 

 thickness of the bands, therefore, shows how much material was per- 

 manently accumulated during the period of the ripples, which must 

 be a portion of time so definitely connected with the structure and 

 character of the ripples that I feel persuaded we shall ultimately be 

 able to deduce from them the actual period for any given instance, 

 and thus, knowing how much was permanently deposited in a given 

 time, we should know the rate at which deposition took place. 

 Hitherto I have made so few trustworthy experiments on this point, 

 that I do not profess to be in a position to solve this problem with 

 approximate accuracy, but even now we can form a good opinion 

 respecting the relative rate of deposition, and can perceive that this 



