ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXI 



they do take place, being restricted to limited portions of the elevated 

 area. Secondly, movements extending over comparatively small areas, 

 frequently producing violent local disruptions, and always attended 

 by outpourings of igneous products at numerous points over the vfhole 

 elevated area, I do not mean that these tvro kinds of movements 

 are necessarily referable to causes of different kinds, but possibly to 

 causes of the same kind acting under diiferent conditions, and there- 

 fore producing phsenomena vfith distinctive characters vphich it is 

 expedient to bear in mind in the discussion of these phsenomena. 

 Thirdly, those movements of depression vrhich must have taken place 

 very slowly and continuously, or by small and frequently repeated 

 steps, during long periods of the deposition of sedimentary matter. 

 That such movements have taken place follows as a necessary con- 

 sequence of the law of distribution of organic beings, which asserts 

 that each class of marine animals can only flourish within compara- 

 tively small limits of depth. "What an enormous depression, for in- 

 stance, must have taken place in the region of S. Wales between the 

 period of the earliest fossiliferous beds and that of the coal-measures, 

 an interval of time during which no great sudden movements can have 

 taken place in that region, as proved by the continuity in the strati- 

 fication throughout that immense mass of sedimentary matter. There 

 may also have been, in like manner, slow and continuous elevations, 

 but of such movements we have not the same demonstrative proof, 

 as of the continuous movements of depression. 



It would seem impossible that the intrusion of the enormous mass 

 of contemporaneoiis igneous matter contained in the Barmouth series 

 should not have been accompanied with some disturbance of the 

 stratified deposits, though, as I have already observed, it was not 

 sufficient to produce any discoverable discontinuity in the stratifica- 

 tion. These eruptions of volcanic matter are doubtless to be referred 

 not to one particular epoch, but to successive epochs during a long 

 lapse of time. Also, anterior to any great and more sudden move- 

 ment, there must have been that enormous continuous descending 

 movement which, during the deposition of the whole Silurian series, 

 must have depressed the lower beds from the depth of a few hundred 

 feet below the surface of the sea to that of several thousands. 



The first movement to w'hich a marked discontinuity can be traced 

 was that of the Caradoc period. It does not appear, however, to 

 have affected any large portion of S. Wales, and in the neighbour- 

 hood of the Berwyns it did not assume the character of a great ele- 

 vatory movement belonging to the first class of those above enume- 

 rated. It seems rather to have belonged to the third class in which 

 a comparatively small unequable depression produced a discontinuity 

 of stratification, but one of only small amount. If we refer a consi- 

 derable part of the elevation of the Longmynd to this period, together 

 with the volcanic eruptions and dislocations from thence to Builth, the 

 movement must there have belonged to the second class of the above 

 cases, and have extended, as thus characterized, over a small area. 



The determination of the exact epoch of this movement is a point 



