Address to the Royal Geological Society of Ireland. 115 



currents ; and if a strand does not collect outside, they force it to 

 do so by the judicious erection of groynes. 



Full details respecting these banks will be found in the writing s 

 of the Dutch engineers. 



Now, as to the assistance which the geologist can give in the 

 matter of laying out sea-banks. The ti-avelling of sea-beaches 

 and the denudation of coast-lines and other related natural opera- 

 tions are phenomena which come within the range of observation 

 of the geologist, and which he is specially drawn to study, as they 

 bear so strongly on various questions of physical geology with 

 which he is concerned. The knowledge of such operations of 

 nature is indispensable for the proper laying out of embankments 

 and for the arrangement and effective placing of groynes. The 

 latter are most important defences against the sea, not only in 

 protecting embankments, but also for the preservation of the 

 coast in those places where the sea is encroaching and slowly 

 destroying good land. Many of the Irish intakes which have 

 been successful in keeping out the sea, are not as profitable com- 

 mercially as they would have been if so much money had not 

 been unnecessarily sunk in making the embankment and if 

 groynes had been judiciously employed. 



The catchment drains and canals of many of the Irish intakes 

 are defectively planned for want of the study of the nature of floods 

 in rivers and streams. The laws of floods teach us that ordinary 

 floods occur nearly every year, extraordinary ones at intervals 

 of about twenty years, while some of very excessive violence 

 may occur at greater intervals. In disregard of these facts the 

 catchment drains and their appurtenances are usually only cal- 

 culated to meet the requirements of ordinary floods ; and extra- 

 ordinary floods when they come are always more or less disastrous 

 to the undertaking. To meet the requirements of extraordinary 

 floods, and to give them a sufficient waterway, it is expedient that 

 the banks of the drains should be at some distance from them, 

 and that the bridges be sufficiently capacious. 



A knowledge of chemico-geology is necessary to bring the in- 

 taken lands into cultivation ; this, however, is a subject of such 

 vast extent that it cannot be considered now. 



To cultivate lands flooded by lakes it is necessary to drain tlie 

 lakes more or less. If a lake is only partially drained, tlic form, 



