OF LITTLE KNOTT (CUAIEEELAND). 515 



Further, the glacier above described belongs either to the earlier 

 period of greatest giaciation or to the later or " valley glacier " period. 

 If to the former, then all theories of vast ice-sheets melt away : if 

 to the latter, we must suppose either that the interval between the 

 two periods was long enough to allow of the accumulation of the 

 needful store of picrite boulders on the flanks of the glen for 

 transportation by the growing glacier of the later period, or that the 

 erosive power of glacier ice is so slight that the shattered ridges of 

 picrite were not materially diminished even in the period of ice- 

 sheets, and that these preserved, without plundering, the stores 

 accumulated in preceding geological periods. 



I deem it needless to prove that these boulders have been trans- 

 ported on and not below the glacier, because the latter mode of 

 transport, at best a rather hypothetical one, appeared to me 

 irreconcilable with the evidence to be obtained on the ground. Hence 

 I regard the district of Little Knott as bearing testimony adverse to 

 the extreme theories of " ice, its extent and work,"' of which during 

 the last twenty years we have heard so much. 



Hornhlende-Picrites of Anglesey and Caernarvonshire. 



In the matter of the Anglesey picrites, I have obtained some 

 further information. In 1883 during an afternoon's walk in the 

 district between Ty Croes and the sea, I observed whole or broken 

 boulders, not less than five in number, of which some details are 

 given in the report of the Erratic Blocks Committee of the British 

 Association * ; but I felt some doubt as to the correctness of my 

 identification of the rock in the supports of the Cromlech Barclodiad- 

 y-gawras. (It will be remembered that here only the weathered 

 surfaces of the stones can be examined.) But I have received from 

 Prof. Hughes a supply of specimens from two masses of very similar 

 rock which he has discovered in situ in Anglesey. As my engagements 

 during the last eighteen months have made it impossible for me to 

 visit that island, I am indebted to his kindness for the following 

 information. The one rock occurs at Caemawr near Llanerchymedd, 

 the other at Pengorphwysfa, 1| mile E.S.E, of ximlwch, both 

 localities lying much in the same line, rather to the N. of jN'.E. of 

 the district over which the boulders are scattered. 



Prof. Hughes thus writes : — "With regard to the mode of occurrence 

 of the Caemawr dykes, from which, I think, most of the boulders of 

 Central Anglesey are derived, there is not much to say. They are 

 a group of dykes of various composition and texture, occurring in 

 the Arenig series a little below the zone of Didymograptus Murchisoni. 

 Their trend, as seen on the surface, is approximately with the strike 

 of the rocks, but I have not ascertained whether or not they 

 generally coincide with the bedding for any distance. There are 

 -differences of texture between neighbouring dykes, and still greater 

 differences, sometimes, between adjoining parts of the same dyke. 

 The very light-coloured more coarsely crystalline specimen (see 



* Volume for 1883 (Southport), p. 146. 



