86 PKOCEEDINGS OF THE NATIONAL MUSEUM vol.76 



THE ASHGILLIAN OF NORTHWESTERN ENGLAND 



Marr's term Ashgillian does not appear on the chart, but since 

 I enjoyed an opportunity the past summer to study the typical and 

 best known exposures of the series in northwestern England under 

 the able guidance of T. C. Nicholas and W. B. R. King, both of 

 Cambridge University, some expression of my opinion concerning 

 the age of the beds covered by the term seems desirable. As defined 

 by Marr ^^ and as thie beds and fossils appeared to me in the field, 

 the Ashgillian should fall within the Medinan epoch. Whether 

 any part of the series is of Richmond age I am not prepared to 

 say, but the upper part at least I am strongly inclined to refer to 

 the Alexandrian. 



Let me say further that at no place visited by us in 1929 did we 

 see any convincing contact between beds admitted by our guides to 

 be very low Silurian and beds of Ashgillian age that my British 

 friends classify as " Upper Bala " or " Caradocian " and therefore 

 as " late " Ordovician. The supposed " contacts " and sometimes 

 " passage " beds that were pointed out as marking the transition 

 from the Ordovician to the Silurian in no case presented the dias- 

 trophic criteria and qualities that in America we insist on being 

 definitely located in the outcrop and indubitably shown to be present 

 at the Ordovician- Silurian contact. However, much more convinc- 

 ing and diastrophically well marked contacts occur in the Lakes 

 District and elsewhere in Britain between lower beds, but as their 

 fossils consist mainly of Ordovician generic types and perhaps par- 

 ticularly because they lack monograptids they are referred by the 

 British geologists to the Ordovician system. 



In my opinion the naturally defined base of the Silurian in the 

 English Lakes District lies at the base of the Coniston limestone 

 series. This series begins with the " Stile End beds," to 50 feet 

 thick and consisting of sandstones, grits, and as much as 10 feet of 

 coarse conglomerate at the base. The Stile End beds are succeeded 

 by the Applethwaite beds — calcareous shales, banded and nodular 

 limestones — about 100 feet thick, with a basal zone full of pebbles 

 derived from the underlying Borrowdale volcanic series. Here and 

 there the Applethwaite limestone is highly fossiliferous, the fauna 

 consisting mainly of corals. But these corals — among them several 

 species of Heliolites — are of kinds that viewed in the light of Ameri- 

 can occurrences could indicate nothing older than topmost Medina 

 or Clinton. The Applethwaite is succeeded by Marr's Ashgill group, 

 70 feet thick, with the Phillipsinella beds at the base and the Phacops 

 mucronatus beds — now admitted by Troedsson to be Silurian — in 



'•Marr, J. E., The Lower Paleozoic rocks of the Cautley District: Geol. Soc. London 

 Quart. Journ., vol. 69, p. 5, 1913. 



