408 C. A. Raisin — The Serpentines of the Lleyn. 



Where, on the contrary, the materials are coarse, we may suppose 

 the rise to have been more rapid and the velocity of the current 

 greater. When, again, large blocks have been transported, a more 

 energetic movement is made manifest." What rate of tide-run is 

 sufficient to move about " large blocks " ? " Some indication of the 

 duration of the uplift is afforded by the mass of material moved, and 

 the distance traversed." Comparison with observed effects of tides 

 seems to show that to move large blocks the duration must be very 

 short if the uplift be moderate, or the uplift extremely gi-eat if the 

 duration be moderate. The Tides also teach that high speeds are 

 generated only in confined channels. The water pours through the 

 Swinge, the Kace of Alderney, the Gouliot Pass, or the Sound of 

 Lihou ; but in the open bay of St. Aubin's a thirty feet fall does 

 not create any remarkably strong current off the shore. 



There are two classes of catastrophe whose effects have probably 

 magnified our ideas as to the potency of rapid rises or falls. When 

 reservoirs or other dams burst, as in the St. Gervais calamity lately 

 filling the newspapers ; when earthquake waves roll in on to the 

 land, as after the Krakatao eruption, direful disaster has been wrought. 

 But neither catastrophe is properly comparable with the cases we 

 have been considering. A reservoir is usually hundreds of feet 

 above the sea-level. If its dam gives way the water flows off down 

 this distance : the results, therefore, are analogous to what would 

 happen if the land were elevated these hundreds of feet instan- 

 taneously. The earthquake wave, too, sweeps up the shore at a rate 

 far exceeding what we can call ' rapid ' ; it rises its thirty or forty 

 feet in a few seconds. Also its analogies belong to the case of a 

 sinking, not of an uplifted, land. 



We accordingly conclude that the effects of * Rapid ' emergence 

 of land from below the sea will not differ from what we see 

 happening daily between high water-mark and low, unless the 

 rapidity is vastly greater than twenty or thirty feet in six hours ; 

 whilst even this moderate rapidity far transcends the powers of any 

 agents with which we are at present acquainted. 



IV. — The So-called Serpentines of the Lleyn. 



By Catherine A. Eaisin, B.Sc. 



IN the south-west of the Lleyn peninsula, the country inland is 

 generally covered by drift, but the Survey map marks some 

 isolated patches of rock as " Serpentine." When I first visited the 

 district, with the kind encouragement of Professor Bonney, under 

 whom I was continuing my work at University College, I made a 

 collection of these so-called " Serpentines " from ten different 

 localities. The one at Perth din lleyn had been shown by 

 Professor Bonney to be mainly diabase,^ and others of the examples 

 have been since described in a previous number of this Magazine 



' On the Serpentine and associated Eocks of Anglesey; Avith a Note on the 

 so-called Serpentine of Forth din lleyn. By Prof. T. G. Bonney, Q.J.G.S., 1881, 

 vol. xxxvii. p. 48. 



