part 2] PALEOZOIC eocks of the llangollen district. 177 



Prof. O. T. Jones, Mr. Philip Lake, and Mr. John Pringle for the 

 determination of fossils — a difficult task with material so distorted 

 by cleavage. To Dr. H. H. Thomas we are greatly indebted for 

 his examination of some of the more difficult rock-types. 



II. Historical Summary. 



Pending the publication of the new Greological Survey map, 

 upon which the map (PI. V) is based, the reader is referred to the 

 Old Series map, Sheets 74 N.E., N.W., & S.E., in which the major 

 features of the structure are well displayed. This, it is believed, 

 was largely the work of T. W. Aveline and J. B. Jukes. 



More recent research has been confined, with few exceptions, to 

 the southern part of the area, that is, between the Dee Valley and 

 the Berwyns. 



Our present knowledge of the succession is largely due to the 

 work of Dr. T. T. Groom & Mr. P. Lake,^ whose paper on Grlyn 

 Ceiriog contains a full bibliography. 



The zonal classification of the Salopian was described, in the 

 case of the Wenlock by Miss O. L. Elles,^ and in the case of the 

 Lower Ludlow Series by Dame Ethel Shakespear (nee Wood).^ 



The igneous rocks of the Berwyns have been redescribed in a 

 posthumous paper by T. H. Cope."^ 



North of the Dee the geology of the Lower Palaeozoic rocks has 

 been barely touched, although reference may be made to early 

 work by the late Prof. T. McK. Hughes ^ and to the maps made 

 by Sir Aubrey Strahan ^ ; also to his description of the area lying 

 north of Llandegla. The work of Gr. H. Morton was confined to 

 the Carboniferous rocks. 



During the recent survey, brief notices were published in the 

 ' Summary of Progress ' of the Geological Survey for the years 

 1911, 1912, & 1913. One of us has also given a sketch of the 

 geology of the Llangollen district.^ 



^ T. T. Groom & P. Lake, ' The Llandovery & Associated Eocks of the 

 Neighbourhood of Corwen' Q. J. G. S. vol. xlix (1893) pp. 426-39 ; T. T. 

 Groom & P. Lake, ' The Bala & Llandovery Rocks of Glyn Ceiriog ' ibid. 

 vol. Ixiv (1908) pp. 546-95 ; and P. Lake, ' The Denbighshire Series of South 

 Denbighshire' ibid. vol. li (1895) pp. 9-22. 



2 ' The Zonal Classification of the Wenlock Shales of the Welsh Border- 

 land ' Q. J. G. S. vol. Ivi (1900) pp. 370-413. 



^ ' The Lower Ludlow Formation & its Graptolite- Fauna ' ibid. pp. 415-91. 



"* ' On the Igneous & Pyroclastic Rocks of the Berwyn Hills ' Cope 

 Memorial Vol. Proc. Liverpool Geol. Soc. 1915. 



= Q. J. G. S. vol. xxxiii (1877) p. 207. 



^ ' The Geology of the Neighbourhoods of Flint, Mold, & Ruthin ' Mem. 

 Geol. Surv. 1890, pp. 4-6. 



7 L. J. Wills, ' The Geology of the Llangollen District ' Proc. Geol. Assoc, 

 vol. xxxi (1920) pp. 1-15. 



Q. J. G. S. No. 310. N 



