518 Proceedings of the Royal Irish Academy. 



No. 66.— SOUTH-WEST COAST OF AUSTEALIA. 



Tasmania, . 

 New Munster, 

 Gambar Is., 

 Central America, 

 Cuba, 



North America, 

 Spain, 



Sardinia, . 

 Soutliern Italy, 

 Candia Is. 

 Ked Sea, 

 Arabia, 



South point. 



From Milford Sound to Ashburton. 



Paumoa Group. 



Gruatemala, Quisaltenango to C. Catoche. 



North-west Coast. 



Florida Point. 



Pyrenees, and Santander Coast. 



Straits of Bonifacio. 



Catanzaro. 



Head of Gulf of Acaba. 

 South Coast, Dofar. 



These details show that the great circles marked on the map of 

 Europe, in connexion with earthquake disturbances, traverse outside 

 Europe, countries and districts more or less markedly subject to earth- 

 quakes, or volcanic in their nature, and therefore it would be possible 

 to refer to such great circles the several earthquakes occurring on their 

 directions. This would be a first method of grouping, and would allow 

 of comparisons as regards contemporaneity, or, what is of equal interest, 

 regularity of interval between the shocks occurring at points distantly 

 removed. 



The referring of earthquakes to great circle directions would, 

 furthermore, facilitate the definition of those large extents of globe 

 surface affected by great earthquakes, such as that of Lisbon in 

 1755, which can be very conveniently defined in this manner. Such 

 earthquakes, and indeed many others, have usually been represented 

 as extending over an ellipsoidal or circular space. This mode of defi- 

 nition can only be approximative, and when the limits are at all 

 accurately determined, the surface affected usually presents a polygonal 

 form. Thus the great earthquakes of 1811-13 are marked on Berg- 

 haus' map as extending over a triangular space, having for summits 

 the Azores, the valley of the Ohio, and !N^ew Granada, that is to say, 

 a polygonal form capable of being defined by segments of great circles. 



The northern side of this triangle corresponds fairly with the great 

 circle (No. 55) axis of Bay of Eundy, which runs from Madeira to 

 Omaha (Nebraska State), about the extent of that side of the triangle. 

 It might, therefore, be correct, and would be convenient, to refer and 

 define earthquakes relatively to the great circles along which or near 

 the intersections of which they occur. This I have done in the present 

 memoir for the Central Saxony earthquake district, so remarkably 

 connected with the great circle directions No. 50 and No. 2, and 

 parallels. 



I have also taken the records for 1870-78, and referred the suc- 

 cessive earthquakes to great circle directions with very interesting 

 results ; and in certain cases, where the limiting points are given, the 

 concordance of the boundaries of the locality affected, with the coast- 

 line directions passing thereat, is very remarkable. 



