238 T. M'KENNY HTJGHES on the geology of ANGLESEY. 



To work the section upwards, we notice that we always have the 

 quartz-jasper conglomerates above the gneissic rocks. There is 

 never any group like the gnarled series associated with the beds 

 that immediately succeed the gneiss ; but the quartz-jasper conglo- 

 merates pass by alternations of beds into grey sandstones, which 

 weather brown or yellow, as may be seen in Porthlygan on the 

 N.E. coast and along the N.W. flank of the central axis from 

 Llanerchymedd to near Llanfaelog. This is clearly the " grit with 

 multitudes of a dwarf variety of Ortliis calligramma " that Salter spoke 

 of. But the species is 0. carausii, a plano-convex shell of smaller 

 size and with fewer ribs than 0. calligramma^ and a characteristic 

 Tremadoc fossil ; I found also near Tyhen Neseuretus ramseyensis, 

 another Tremadoc form. They exactly resemble specimens from the 

 Tremadoc of Eamsey Island in the character of the rock and mode 

 of preservation of the fossils. 



The Tremadoc sandstones are succeeded by black slates, into which 

 they often pass up by alternations, as may be seen along the coast N. of 

 Porthlygan and beyond the fossiliferous sandstones N. and N.W. of 

 Llanerchymedd. On the S.E. side of the central axis there is not so 

 clear an exposure of the conglomerates and Tremadoc sandstones. 

 Brown sandstones, however, occur near Llangwyllog church, and in a 

 quarry a little to the south of it. Eesting on these sandstones are black 

 shales from which I procured a small fragment of a Trilobite and a 

 number of Graptolites, most of which are common forms in the 

 Arenig. These black shales twist round and are seen in the rail- 

 way-cutting E. of Llangwyllog Station to be faulted against the green 

 gnarled series. Mr. Lap worth has looked over the Graptolites for 

 me, and refers them to the following species : — 



Dicellograptus, n. sp. allied to D. Climacograptus cselatus, Lapw. 



sextans, Hall. Glossograptus ciliatus, Emmons. 



Diplograptus dentatus, Brongn., sp. And some new forms of Diplograptus ^ 

 ' foliaceus, Murch. Glossograptus. 



Climacograptus confertas, Lapw. 



Hardly any of the specimens are sufficiently well preserved to show 

 all the specific characters ; therefore it is possible that when we have 

 collected a larger number some of these determinations may be shaken, 

 and it is probable that the list will be largely added to. 



Erom the evidence before him, Mr. Lap worth is inclined to refer 

 the bed to the middle part of the " black-shale group," expressing 

 some doubt as to whether they should be called Highest Arenig or 

 Lowest Llandeilo. The fragment of a Trilobite consists of three 

 body-rings ; and from the number of these in a given space, and the 

 relative breadth of pleurae to axis, it agrees very well with Tri- 

 nucleus Murchisoni. 



On the whole I am inclined to refer the Llangwyllog beds to the 

 Arenig, and expect to find the Llandeilo forms by and by in the 

 slates of Llanbabo and Llanelian. 



"We have therefore two horizons, Tremadoc and Arenig or Lower 

 Llandeilo, pretty clearly determined by characteristic fossils. The 

 beds with Graptolites at Llangwyllog rest on brown sandstones, and 

 pass up into black slates or shales. On the west of Llanerchymedd 



