part 1] AoSrJSIVERSARY ADDRESS OF THE PRESIDENT. XCl 



rocks include feeble flows of lava at several horizons, together with 

 thick bedded breccias composed in part of like materials. The 

 lavas are of various types, and seem to have followed one another 

 in order of decreasing basicity. There are olivine -basalts containing 

 orthoclase, rocks intermediate between these and olivine-trachytes, 

 and again trachytes without olivine but rich in biotite and approxi- 

 mating to the minette and cuselite types. The series may be 

 carried farther if we include the rocks of Cawsand and Withnoe, 

 near Plymouth Sound : the one a biotite-trachyte and the other, 

 which has intrusive relations, a quartz -porphyry in which the 

 felspar is almost wholly orthoclase. Here then is a well-marked 

 series of rocks of strongly alkaline nature, rich not in soda but in 

 potash. Sir Jethro Teall has compared them with some of the 

 Permian lavas of the Continent, but in British petrography they 

 are unique. The origin of highly potassic rocks in general is a 

 problem upon which more light is needed, but it could not be 

 profitably discussed in connexion with this isolated occurrence. 



Throughout the European area the time during which the Meso- 

 zoic strata were laid down, though marked by many vicissitudes, 

 was not interrupted b}'' any crustal disturbance of an accentuated 

 type. At a later epoch all Southern Europe was involved in the 

 Alpine crust-movements, with attendant consequences which are 

 not yet exhausted. Some outlying ripples of this system reached 

 the British area, but there was no igneous outbreak in this country 

 which can be probably related to those events. Tertiary igneous 

 action in Britain has a different significance, and its seat was not in 

 the south but in the north and north-west. Affecting an area of 

 which the northern half of Britain is but a small fraction, this 

 immense and widespread activity was not connected with any 

 lateral creep of the earth's crust, nor did it coincide with mere slow 

 subsidence within the limits of geosynclinal troughs. Its relations 

 seem to be with movements of a larger order. If we follow 

 Marcel Bertrand, we may connect this extra-Alpine Tertiary igneous 

 activity with the foundering of extensive tracts in the Atlantic 

 region, a deepening of an oceanic basin which was itself of no great 

 geological antiquity. It is clear at least that the important move- 

 ments in this region were differential movements in the vertical 

 sense, expressed by normal faulting instead of folding, and there- 

 fore in their tectonic effects plateau-building rather than mountain- 

 building. 



