A. D. N. Bain — The Age of the Milhurn Grotq^. 261 



stronger than any one of them. For that reason the eustatic 

 hypothesis has seemed worthy of statement. The search for proof 

 of eustatic shifts during more remote periods might be easier if 

 even a small modern movement of the kind were proved, especially 

 if its cause were also determined. 



Note on the Age of the Milburn Group. 



By A. D. N. Bain, B.Sc. 



f\N the western side of Cross Fell lies a group of beds consisting 

 ^^ of andesitic lavas and ashes, with intercalated beds of shale. The 

 shales are tranquilly bedded, testifying to their having been deposited 

 in quiet water. It has therefore been inferred that the igneous 

 material is volcanic, and was poured out on a sea-floor. The beds ' 

 were first described by Harkness,^ later by Goodchild,^ who gave the 

 name " Milburn Group ", and subsequently by Professors Nicholson 

 and Marr.^ 



The exact age of these beds could only be approximately 

 determined because of the scarcity of determinable fossils obtained 

 from them. Goodchild recorded Didyinogra/ptics murcMsoni (Boeck), 

 and Professors Nicholson and Marr recorded Diplograptus dentatus 

 (Brongn.).® Professor Marr assigned them to either the Lower 

 Llandeilo or the uppermost part of the Arenig, at the same time 

 expressing a hope " that local observers will pay special attention to 

 the fauna of these beds, as the exact determination of their age 

 is a matter of considerable importance ".* Later Goodchild grouped 

 them with the Ellergill Beds, a grouj) of earthy slates underlying 

 the Milburn Group and of about the same age, as being equivalent 

 to the lower part of the Volcanic Series of Borrowdale.^ 



It is in Wythwaite Hole that these beds are best exposed and 

 where they have been generally studied. A good exposure occurs 

 in a similar " hole " to the east of Wythwaite Hole proper, where 

 a band of shale is exposed, quite regularly bedded and lying between 

 masses of andesitic lava. It is about 3 feet thick and dips to the 

 south-east. The lavas above show regular parallel banding, dij)ping 

 at a greater angle in the same direction. 



From this band of shale the following graptolites have been 

 collected by the author ^ :— 



^ Quart. Journ. Geol. Soc, vol. xxi, 1865, pp. 235, 340-1. Cf. Harkness, 

 Quart. Journ. Geol. Soc, vol. xix, p. 127. 

 2 Proc. Geol. Assoc., vol. ix, No. 7. 



* " The Cross Fell Inlier " : Quart. Journ. Geol. Soc, vol. xlvii, 1891, 

 p. 502. 



* Geol. Mag., 1894, p. 128. 



^ The Geology of the Country between Appleby, Ullsivater, and Hawes 

 Water. Mem. Geol. Surv. England and Wales, Sheet 30 (New Series), 

 102 S.W. (Old Series), pp. 34 and 39. 



^ A Phyllograptus, the species of which has not yet been determined, has also 

 been obtained. 



