TRANSACTIONS OF SECTION C. 547 



WEDNESDAY, AUGUST 21, 1878. 



The following Papers were read : — 



1. On the Age of the Crystalline Rocks of Donegal. 

 By Professor W. King, D.Se. 



2. On the Correlation of Lines of Direction on the Globe, and particularly 

 of Coast Lines. By Professor J. P. O'Reilly. 



The forces which have acted from the interior of the glohe have been the 

 most important of those which have produced the present distribution of land and 

 water at its surface. 



Those forces are more essentially, contraction due to secular cooling, and gravi- 

 tation. Furthermore, the mineral constitution of the earth's crust has necessarily- 

 influenced the mode of action of those forces, and assuming that constitution to 

 be homogenous, and to be represented by the lavas modern and ancient, we might 

 expect that the contraction forms presented by those lavas when in great mass 

 would represent the forms to which secular contraction gives rise at the surface of 

 the earth, and that these would be limited by lines of direction presenting poly- 

 gonal forms somewhat as the contraction forms of the older lavas (basalts and 

 trachytes). Those lines of direction would represent the joints or fissures along 

 which the interior forces have exercised their greatest action, that is, those lines 

 which are represented on the globe by mountain chains. Now these influence 

 notably the forms of the continents as represented by the present distribution of land 

 and water. On the other hand, the boundaries of seas past and present have been 

 and are simply coast lines determined by the intersection of the sea surface with 

 the upraised strata, whose direction is essentially represented by that of the adjacent 

 mountain chains, and of the system of fissures or joints forming their axes. Thus 

 mountain chains and coast lines can be correlated. 



Having measured a series of blocks of basalt at the Giant's Causeway, I have 

 found that the dominating polygonal angle, or that one which most frequently 

 occurs, is the angle 110°, and its supplement 70°; moreover, that the form presented 

 by an isosceles triangle having 70° and 70° at the base, and 40° at the summit, is 

 markedly present in the basalt columns of Fair Head. This form I had pre- 

 viously recognised as continually occurring on maps, especially geological, and 

 have thereon based a system of correlation of lines of direction. 



Adopting as line of departure, the direction of the East Coast of Madagascar, 

 and taking this from Imray's most recent chart, I have found that the greater 

 number of lines of direction of coasts and mountain chains on the earth's surface 

 can be derived therefrom solely by the intervention of the angle 40° and its mul- 

 tiples, and 70°, and as proof have transferred from the globe those lines to maps, 

 both geographical and geological, and found such lines to be related with many 

 others on the map through the intermediary of those angles, and that this law is 

 general. Amongst the many maps which might be exhibited in proof thereof, I 

 would mention Haidinger's Geological Map of Austria, whereon occur marked lines 

 of direction which can be most distinctly reduced to this law, as also the maps of 

 the Geological Survey of Great Britain and Ireland. 



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