Dr. J. BE. Marr—The Ashgillian Series. abe) 
IJ.—On tue Asueriiran. SERIES. 
By J. E. Marr, Sc.D., F.R.S. 
N my Address to the Geological Society in 1905 I proposed the use 
of the term Ashgillian for the uppermost series of the Ordovician 
System, but did not discuss therein at length the exact relationship 
of the beds to which the term was applied, with the underlying and 
overlying strata. An examination of the literature treating of these 
rocks showed that there was a considerable amount of confusion with 
regard to these strata, and it is the object of this paper to clear this. 
up and to indicate what deposits are actually referable to the 
Ashgillian division. 
I. Hisvrorticat. 
In the Introduction to the ‘“‘Synopsis of the Classification of the 
British Paleozoic Rocks” (which is undated, though a postscript is 
dated July 19th, 1855), Sedgwick gives the Bala Group as constituting 
the whole of his Upper Cambrian, while the Arenig slates and 
porphyries are placed in the upper division of his Middle Cambrian. 
He divides the Bala Group as follows:—‘‘a. Lower Bala rocks. 
(1) A great series of dark and sometimes earthy slate. (2) A great 
series of slates, flags, and grits, ascending to the Bala Limestone.— 
b. Upper Bala rocks. Under this term are included: (1) The Bala 
and Hirnant Limestone and the Llandeilo calcareous flag. (2) Flag- 
stones, slates, calcareous beds, and shelly sandstones of Caer Caradoc. 
The whole series ending, in North Wales, with slate and flagstone ; 
and, in South Wales, with slates, grits, and coarse conglomerates. 
The group of very great thickness.”’ 
In this scheme the term Upper Bala was used in a wider sense than 
that in which it was subsequently used by Salter with Sedgwick’s 
consent. 
In ‘‘The Catalogue of the Collection of Cambrian and Silurian 
Fossils in the Geological Collection of the University of Cambridge,” 
published by the University Press in 1873, the term ‘‘ Upper Bala”’ 
was restricted, and a new term ‘Middle Bala’’ introduced and 
adopted ‘‘for the Bala Limestone and its associated sandstones and 
slates, several thousand feet thick in N. Wales 
On p- 39 of the Catalogue the following is given as the extent of 
the Middle Bala Group: ‘‘ Professor Sedewick only includes in this 
group the 9,000 feet of beds, chiefly arenaceous, slaty, and with some 
calcareous bands, which lie over the dark earthy slates of the Arenig 
section (Lower Bala). The group extends a short distance, probably 
a couple of hundred feet, above the Bala Limestone. But it does not 
-include the Hirmmant Limestone, which is the base of the Upper Bala 
group next described. It appears to represent in mass the whole of 
the Caradoc Sandstone proper, in Shropshire.”’ 
We here find a definite statement that the Hirnant Limestone is the 
base of the Upper Bala Group. 
On pp. 72-83 the Upper Bala fossils are catalogued, and at the 
head of p. 72 the following grouping is given :— 
