Vol. 67.] ANNIVERSARr ADDRESS OF THE PRESIDENT. 1XXX1X 



more complete knowledge of the detailed succession and nature of 

 the movements of each period. 



So far as time is concerned, extrusive igneous action has been 

 associated mainly with the close of geosynclinal conditions, while 

 intrusive, and particularly plutonic, activity has been connected 

 with the formation of geanticlines and orogenic uplift. The 

 variation which shows itself in the products of a single petro- 

 graphical province seems to be, in the main, due to differentiation ; 

 and we owe to Mr. G. Barrow and Mr. A. Harker the valuable 

 suggestion that one of the chief determining factors in the progress 

 of differentiation is the action of varying pressures on a magma in 

 a state of partial or of potential fluidity. This suggestion, though 

 not yet fully worked out, promises to be a most fruitful one, and 

 holds out hopes of establishing a closer relationship than has 

 hitherto been possible between the structures of the earth-crust 

 and the nature and distribution of igneous material in connexion 

 with it. 



Mr. Finlayson has recently attempted to work out the relation 

 between the periods of earth-movement and the formation of 

 ore-bodies. He finds that only a small proportion of our metalli- 

 ferous veins date back to the Caledonian or earlier movements, but 

 that the great period of vein-filling was associated with the 

 post-Carboniferous movement, even in areas where the country 

 rocks are of Ordovician or of Silurian age. Ores found in post- 

 Carboniferous rocks have in most cases been deposited there by 

 redistribution, concentration, and enrichment from earlier deposits. 

 If he is right in his contention, he has given to the Later Palaeozoic 

 Era a further claim on our gratitude for its contributions to the 

 mineral wealth of our country. 



The connexion is probably only indirect between the several 

 movements which have affected the British Isles and their present 

 systems of drainage and consequent relief. "While the Wealden 

 dome is drained by a system of rivers that dates to the Alpine 

 movement, and the Thames and Solent vallej 7 s are consequent on 

 structures due to that movement, phenomena apparently similar 

 in Wales, Lakeland, and the Pennine Chain cannot be similarly 

 linked with older movements. The number of submergences and 

 planations, the covering with sediment and the subsequent 

 removal of it, have probably destroyed or buried lines of drainage 

 due to earlier movements, and such relationships as still subsist 

 are due to that adjustment of drainage to structure which must 

 always work itself out eventually, on whatever lines it may make 

 vol. lxvii. y 



