494 ME. W. B. E. KISG OX THE [vol. lxxix, 



at this horizon. The lithology of the shales and the presence of a 

 phosphate-hand point to Avaters devoid of normal terrigenous 

 sediments, while the manner in which the shales come immediately 

 above, and even interstratified with, very shallow-water deposits 

 show that they cannot be of deep-water origin. Thus the beds 

 were formed in a shallow sea, free from much sediment, yet 

 unfavourable for the normal life of the times : a state which may 

 probably be attributed to lagoon conditions somewhat similar to 

 those described by Mr. E. E. L. Dixon in the Carboniferous rocks 

 of Grower. 1 



Caradoc-Ashgill junction. — At localities where the black 

 shales can be followed up into the overlying strata, as at Pen-y- 

 garnedd and at Aber Marchnant. the passage is seen to be gradual, 

 and there is no sign of unconformity, 3 the black shales merely giving 

 place to dark blue-grey and black blocky mudstones which yield 

 the typical basal Ashgillian fauna that will be noted below. From 

 this it is clear that the black shales occur between the lower part 

 of the Orthis-actonice Zone and the base of the Ashgillian : that 

 is, they lie on the horizon of the upper part of the Orthis-actonice 

 Zone of Bala, and therefore represent beds of limestone and 

 calcareous ash on the western side of the Central Wales syncline. 

 The recent work by Miss Gr. L. Elles 3 rules out any possibility of 

 unconformity between the Orthis-actonice Beds and the basal 

 A.shgillian (Rhiwlas Limestone), and therefore the Pen-y-garnedd 

 Shales cannot be an horizon that is not present at Bala. It may 

 be noted also that the limestones at this horizon at Bala are some- 

 times phosphatic and oolitic,* and were certainly formed in shallow 

 water. 



Ashgillian. 



Lower Phillipsinella Bed s. — Simultaneously with the 

 change from black shales to dark blocky mudstone there is a 

 marked change in the fauna. Instead of graptolites and dwarfed 

 inarticulate brachiopods, an extremely rich and varied fauna makes 

 its appearance. This fauna contains all the tj'pical elements of 

 that of the basal Ashgillian. 



In the old quarry 150 3^ards north-east of Aber Marchnant Farm 

 («1, fig. 2) dark calcareous blocky mudstones have yielded a large 

 number of excellently preserved fossils. One of the most striking 

 features of the fauna is the small size of the forms and the manner 

 in which numerous individuals of the same species are found in 

 close proximity one to the other : for example, one specimen 

 showing a surface of about 2 square inches shows fragments of the 

 heads of four individuals of a small variety of Lichas laxatus 

 M'Coy, while another specimen having a surface of about 1 square 

 inch is practically composed of the heads of Galymene. 



1 E. E. L. Dixon & A. Vaughan, Q. J. G. S. vol. lxvii (1911) pp. 525-31. 



2 ' Summary of Progress for 1919' Mem. Geol. Surv. 1920, p. 5. 



3 Q. J. G. S. vol. lxxviii (1922) p. 152. 



4 Ibid. p. 142. 



