90 Remarks on a Tertiary Deposit 



better to resist the wear of water. Instances of this concre- 

 tion are very common where the action of the spray has not 

 destroyed the surrounding matrix. At one cliff out of reach 

 of the sea, where portions of the rock have fallen away, con- 

 cretions are very numerous. The sides of the rock are 

 covered with them running through the strata like roots, or 

 hanging down from the roof so as easily to be mistaken for 

 stalactites, if they were not a little too crooked and irregular. 

 Their appearance is just that of bent coral, about half an 

 inch or more in diameter. The outside of these concretions 

 is just like the rock itself, that is, are composed of small 

 fragments agglutinated together ; but on breaking them the 

 inside is found to be hard and compact, like cherty lime- 

 stone or dolomites. They are usually formed in concentric 

 rings. I do not suppose that the action of the water in 

 causing them has been merely mechanical. I suspect, from 

 the large quantity of magnesia contained in them, that a 

 doubly basic salt of carbonate of lime and magnesia is formed 

 by chemical decomposition. Slow filtration of water might 

 alone be a sufficient cause, because it is certainly from some- 

 thing of this kind that the layers of flint in the Mount Gam- 

 bier limestones are chiefly owing. This is a department of 

 geology where investigation is much required, for the " pot 

 stones " in the chalk at home, which owe their origin to fil- 

 tration of some kind, are by no means clearly accounted for. 

 In addition to the corrosive action just described, the wearing 

 of the strata by waves is very considerable, and thus we may 

 see that the ocean is here indemnifying itself for the losses 

 occasioned by the upheaval of the land. There can be no 

 doubt that the sea will not be long destroying the beds within 

 its reach, if the work of destruction proceeds as quickly as it 

 has within a comparatively short space of time. We may, 

 therefore, witness two phenomena not often associated to- 

 gether, namely, the land rising and the sea encroaching 

 rapidly. It is interesting to observe how the sea soon re- 

 places what it removes, and the seam of limestone which tops 

 the rocks unconformably, answers the question which may be 

 asked : what has become of the immense masses of rock which 

 have been already destroyed ? Such, for instance, as those 

 portions which must have joined the coast with the rocky 

 islands which fringe it. However, no conception of the great 

 work of denudation which has taken place can be gathered 

 from the comparatively small ravages in Guichen Bay. My 

 belief is that the whole coast, perhaps as far as the mouth of 



