176 DE. WILLS AND ME. SMITH ON^ THE LOWEE [vol. Ixxviii, 



4. The LowEE Paljeozoic Eocks of the LLAifooLLEif Disteict, 

 ivith special reference to the Tectonics. By Leonaed 

 Johnston Wills, M.A., Ph.D., F.Gr.S., and Beenaed 

 Smith, M.A., F.G-.S. (Eead January 19th, 1921.) 



[Plates III-V — Geological Maps & Sections.] 



Contents. 



Page 

 I. Introduction 176 



II. Historical Summary 177 



III. Stratigraphy 178 



IV. Structures 208 



V. The Pre- Carboniferous Moor 222 



VI. Post-Devonian Modifications 222 



I. Inteoduction. 



The area dealt with in the following pages forms the western half 

 of the 1-inch Ordnance Surve}^ Map, New Series, Sheet 121, with 

 small contiguous strips in Sheets 120 & 137. It was mapped by 

 us in the course of official work for the G-eological Surv^ey, in 

 1912 & 1913. The memoir on Sheet 121 is in preparation ; but 

 both it and the map which it accompanies have been greatly 

 delayed b}^ the War. This applies even more emphatically to 

 Sheet 137. In the projected memoir on Sheet 121, special 

 attention will be devoted to the Wrexham-Ruabon Coalfield. 



In preimring the present paper, we have been influenced by a 

 desire to publish in a concise form^ an account of the older rocks 

 Avhich are more of theoretical than economic importance. We 

 have attempted to describe the district as a geological unit, whereas 

 the Survej^ Memoir must be primarily an explanation of the maj) 

 and of the evidence on which it is based. 



We are, therefore, greatly indebted to the Director of H.M. 

 G-eological Survey for permission to publish the following account, 

 which is divisible into two parts. In the first, we aim at stating 

 new facts and important modifications of the hitherto accepted 

 ideas of the stratigraphical sequence, without traversing in detail 

 the work of previous investigators where their conclusions have 

 been confirmed by us. In the second part, we give a fuller 

 description of the tectonics than the space available in the official 

 Memoir would allow. 



Our sincere thanks are due to Mr. C. B. Wedd and Mr. W. B. R. 

 King for help and friendly criticism, and to Miss Gr. L. Elles, 



^ TJie arbitrary limits of the map-sheets are not those of a natural geological 

 region. Accordingly, important parts of the sequence, because they happen 

 to lie in one sheet, have to be omitted in the official description of the other. 



