1850 DE LA BECHE ON ANGLESEY GEOLOGY 171 



and completing the whole. The following letter from 

 De la Beche gave them his impressions of the structure 

 of the ground immediately after they had begun their 

 work : 



LONDON, nth November 1850. 



MY DEAR RAMSAY Touching the mica-slates, chlorite-slates, and 

 other matters of the lower ground in Anglesey, they are, of course, 

 what they can be proved to be ; and no matter what they may be, 

 let us get at the fact. Pray keep a bright look-out for the con- 

 glomerates ; they are most valuable in such investigations. You 

 have probably examined that beneath so much of the Cambrians as 

 is to be seen on the banks of the Menai, near Bangor. The con- 

 glomerates nearer Llanberis show clearly that the matter of the 

 Cambrians there is, in part at least, compounded of older detrita 

 rocks kinds of quartz-rock being among them. If it be really 

 right that the Bangor beds are these said affairs brought up again, 

 probably similar pebbles will present themselves. Here, then, we 

 have evidence of detrital beds consolidated before so much of the 

 Cambrians as such conglomerates may form the base of. 



I know not how you have attacked the ground, but if I had been 

 with you, which I very much regret is not the case (there are, how- 

 ever, matters of more pressing importance now under consideration 

 here), I should have made you master of the country at Holyhead 

 Island, and have proceeded across country to Amlwch, though not 

 quite direct. Taking up the black shales (graptolitic) based upon 

 conglomerates of variable character, but sometimes containing pieces 

 as large as one's head, to these succeed a parcel of trappean affairs 

 limestones beautifully laminated ; above these, shales and more 

 arenaceous rocks, sometimes purplish, and so on to the northern 

 coast, where heavy conglomerates with some impure limestones 

 cover all. A better section is no doubt to be obtained on the sea- 

 coast by means of a boat, but such means of conveyance are now 

 (November) out of the question. The two sections confirm each 

 other, some beautiful granite veins and alterations near them requir- 

 ing a little caution. 



At Amlwch the sections are capital, on the coast especially; 

 the Parys mine, a continuation of the graptolitic slates. Near the 

 place with an unpronounceable name, to which I direct thee, there 

 are some capital conglomerates. Pray look the pebbles well over. 

 Henslow called these Old Red ; they are not so. 



The upper purple beds occupy a position very like similar beds 

 in Ireland the highest of the series there known to us in Wicklow, 

 Wexford, and Waterford. The date of the granitic intrusions of 

 Anglesey is clearly that of the Irish country noticed anterior to the 



