462 R. HARKNESS AND H. A. NICHOLSON ON THE STRATA BETWEEN 



ble cavities filled, or partially filled, with rusty peroxide of iron, 

 apparently due to the decomposition of organic remains. 



It is not our purpose to discuss here the position and age of these 

 " Style-End Grassing beds," as they may be called. It may, how- 

 ever, be noted that the fossils which they have yielded are Bala 

 types, such as Cahjmene JBIumenbachii, Brongn., Orthis vespertilio, 

 Sow., and Peiraia cequisulcata, M'Coy. This is of interest as indi- 

 cating that the volcanic activity of which the Lake-district became 

 the theatre subsequent to the deposition of the Skiddaw Slates, 

 continued to prevail, at any rate, up to the later portion of the Bala 

 period. 



Resting, apparently with perfect conformity, upon the Borrowdale 

 rocks a series of deposits occur which we wish more especially to 

 discuss here. 



These deposits may be grouped in the following ascending order : — 



1. Dufton Shales. 



2. Coniston Limestone and Shales. 



3. Graptolitic Mudstones, or Skelgill beds. 



4. Knock beds. 



1. Dufton Shales. 



The " Dufton Shales " are but locally distributed, though they 

 constitute a well-marked group of muddy sediments underlying the 

 Coniston Limestone proper and its associated shales. They do not 

 appear in any recognizable form beneath the main hue of the Coniston 

 Limestone in the Lake-district itself. They do not seem to occur in 

 the Sedbergh district ; nor have they been recognized in Kavenstone- 

 dale or Eibblesdale ; but they are very well developed in the Silurian 

 area which includes the Cross-Fell range. 



Here they are seen in four principal exposures, owing to the folding 

 and faulting of the strata, viz. : — Swindale Beck, near Knock ; Pusgill 

 and Duf ton-Town dykes, near Dufton ; Harthwaite Gill, near 

 Keisley ; and at the Smelt Mill, near Hilton. 



The thickness of the Dufton Shales in these localities probably 

 exceeds 300 feet. They consist of dark flaggy shales with a rough 

 cleavage, sometimes (as in Swindale) having brownish or greenish 

 ashy beds intercalated among them. These shales are readily dis- 

 tinguished from those associated with the Coniston Limestone, being 

 usually darker in colour, and more flaggy, and having a less perfect 

 cleavage. Near their base they have two bands of nodular lime- 

 stones in them ; these are well seen in Swindale Beck. 



The Dufton Shales are richly fossiliferous throughout, this being 

 the case with the ashy beds before referred to, as well as others of 

 the series. 



The fossils are, for the most part, characteristic Bala types ; and 

 the entire derjosit may be regarded as forming, palaeontologically, 

 the base of the Coniston Limestone. 



The following is a list of the principal fossils which have been 

 collected in these beds :— 





