﻿580 DK. T. GEOOiT AND ME. P. LAKE ON THE [NoV. I908, 



y. Steuctuee of the Aeea. 



At first sight, the structure of the area seems to be one of 

 extreme simplicity. Throughout almost the whole of the district 

 there is a nearly uniform dip from south to north, and the beds 

 follow one another in regular order without any visible break. 

 Only in the eastern portion of the region is there any appearance 

 of interruption in the normal succession, and the Tarannon Shales 

 and the overlying Wenlock Beds are represented by the Geological- 

 Survey map as sweeping unconformably over the upper portion of 

 the beds below. But exposures here are very poor, and a closer 

 examination shows that this apparent overlap is due to faulting. 



'Not is much complication introduced by the existence of a series 

 of transverse faults, which for the most part run from north-north- 

 west to south-south-east, and have their downthrow on the eastern 

 side. The only effect of these faults is that all the beds are thrown 

 back towards the south in a series of steps. 



Strike-Faults. 



An examination of the beds in detail proves, however, that this 

 apparent simplicity of sequence is broken by the existence of at least 

 one strike-fault which lies nearly in the plane of the bedding, and 

 consequently has hitherto escaped detection. As this fault is 

 unquestionably the greatest disturbing factor in an otherwise 

 normal succession, we shall begin by describing the evidence that 

 proves its existence. 



Careful collecting of fossils soon shows that the Bala Beds of the 

 Ceiriog Yalley may be divided into an upper and a lower series, dis- 

 tinguished one from the other by their faunas. In the upper beds 

 we find, for example, Hamipora Hochstetteri, Fhyllopora Hisingeri, 

 Ptilodictya, Trinucleus seticornis, Cybele verrucosa, Encrinurus, and 

 Cheirurus. None of these are to be seen in the lower beds, where, 

 on the other hand, we find Tetradella complicata, Trinucleus con- 

 centricus, Asaphus Powisi, etc., which do not occur in the upper 

 beds. OrtJiis elegantula is also especially characteristic of the lower 

 beds ; for, although it is found in the upper beds, the specimens 

 there are always small and few in number. 



The boundary between the two series is perfectly sharp, and can 

 Ife traced with ease for a considerable distance, as shown upon the 

 map (PI. LIII). Near the farm of Dolhir it will be seen that, in 

 crossing the valley, the boundary forms a V, indicating that the 

 surface of separation between the upper and the lower series dips 

 towards the north, at an angle of about 20°. 



So far, there is nothing to show that the two series are not 

 superposed in normal succession, although the abruptness of the 

 palaeontological change is somewhat against this view. 



Near Dolhir, however, the boundary lies at the foot of a line of 

 low cliffs which runs up from the farm towards the limestone- 

 quarry above ; and an examination of the rocks in these cliffs shows 

 that, although the boundary is nearly parallel to the bedding of the 



