504 Proceedings of the Royal Irish Academy, 



memoir^ I endeavoured to show that the direction of coast-lines in 

 general is mainly due to jointing ; that this must be dependent on the 

 mineralogical constitution of the rock or rocks in "which it is developed ; 

 that this constitution, variable at surface, must change in depth ; and 

 in so far as we have any knowledge of it for great depths, may be pre- 

 sumed to be that of the trap-rocks or basalts; that these, when in 

 great masses, frequently present a columnar structure, acquired by 

 contraction ; that the predominating angles of the prisms formed are, 

 as observed at the Giant's Causeway, of 110°, 70°, 40°, and angles 

 resulting from the combination of these. I further argued that secular 

 contraction is admittedly going on in the crust and at the surface of 

 the earth, but only locally, and therefore unequally ; that there are 

 being formed continuously in the earth's crust points of maximum 

 tension ; that when rupture and consequent local contraction take 

 place, there are produced planes of rupture or joints, which necessarily 

 divide off and limit a certain extent of the earth's crust, and give rise 

 to polygonal forms at the surface of the globe ; that these planes or 

 joints being developed in massive rock, presumably of the nature of 

 basalts, and of very great thickness and extent, give rise to polygonal 

 or prismatic forms, presenting angles such as those observed in basalts ; 

 that by reason of the predominance of certain angles these must repeat 

 themselves, and give rise to directions of planes of jointing which at 

 intervals are similar ; and consequently, that prolonging on the earth's 

 surface a system of jointing along which contraction towards the centre 

 has taken place, this system, marked by its direction, should reoccur 

 at intervals, and show a connexion or correlation with other directions 

 through the intermediary of the predominating angles already men- 

 tioned. 



This theory was supported by examples, and subsequently by the 

 determination and comparison of the systems of jointing observable 

 about the Bay of Dublin, which I showed to have manifest relations 

 with the directions of the neighbouring coast-lines, in accordance with 

 the theory submitted.^ 



Such a theory, however, requires repeated and very direct proofs 

 before it can claim acceptance. If well founded, it should admit of 

 being pushed to its logical consequences, and such I take to be the 

 connexion which I propose to establish between coast-line directions 

 for their correlated lines and parallels) and localities noted for their 

 earthquakes. 



As a matter of fact all earthquakes, whatever their origin, must 

 bring into play the systems of Assuring and jointing which have existed 

 in the locality, tending, however, to modify them and to increase their 

 extent and number. 



^ "On the Prismatic Forms of a Group of Basalts, Giant's Causeway." (Eoy. 

 Irish Acad. Transactions, vol. sxvi., part 22, ISTov. 1879.) 



^ " On the Dii-ections of the Main Lines of Jointing observable in the Rocks 

 about the Bay of Dublin, and their Relations with the adjacent Coast-lines." (Roy. 

 Irish Acad. Proceedings, vol iii., ser. ii., No. 5, Dec. 1880.) 



