292 C. LAP WORTH 0>7 THE MOFFAT SERIES. 



purple colour. A small boss of unaltered strata at the entrance to 

 the gully shows the soft pyritous beds of the M.-gregarius zone, with 

 their lines of variegated mudstone and yielding good examples of 

 Monograptus cyphus, M. gregarius (Lapw.), Diplograptus tamariscus 

 (Nich.), D. folium (His.). 



To the north the flagstones and greywackes of the country are 

 underlain by a band of 4 or 5 feet of purple shales, which reposes 

 at once upon the M.-gregarius zone. On the south side of the section 

 a much greater thickness (about 30 feet) of these purple shales 

 occupies a similar position. 



y. Hartfell Spa (PL XIII. Plan K). — Three miles to the eastward 

 of Bittonside we reach the magnificent exposure of Hartfell Spa. At 

 this locality the Moffat Shales occur in the broad mound called 

 Arthur's Seat (2398 feet), which forms the south-western buttress of 

 the mountain of Hartfell (2600 feet). In the flank of this mountain an 

 enormous gash has been eroded, about a quarter of a mile in length 

 and from two to three hundred feet in depth, which forms a striking 

 feature in the view of the ridge for many miles to the south-east. 

 The name of the spot is derived from a mineral spring which rises 

 among the dark shales in the centre of the exposure, and which has 

 long been celebrated for the strength and efficacy of its waters. 



The black shales of the Hartfell band here rise to the surface in an 

 elongated ellipse, or narrow lenticle, about three miles in length. It 

 is about a quarter of a mile in diameter at its centre, thinning away 

 almost to a point at its opposite extremities. It is formed by a 

 single master anticlinal of the Moffat Series, composed of numerous 

 subordinate and very irregular arches. Nowhere to the north of 

 the Moffat valley are the dark shales so greatly folded and shattered 

 as in this locality ; but, on the other hand, nowhere are the fossils 

 so prevalent throughout the beds ; so that the deficiencies in the 

 stratigraphical evidence of the sequence of the strata are amply 

 compensated by the extra facts derived from palaBontological con- 

 siderations. 



Birkhill Shales. 



As in all our former sections, the highest beds rise from below 

 the greywackes at the extremities and along the inner margin of the 

 ellipsoid. At its south-westerly extremity the stream has cut the 

 narrow gorge known as Frizzle's Linn, generally along the line of 

 junction of the grits and mudstones. To the north of the stream 

 at this point the grits are visibly in conformable contact with a 

 thin group of drab, grey, and black shivery mudstones, similar to 

 those of Bittonside and Headshaw Linn, but in a perpendicular or 

 even slightly reversed attitude. Fossils are numerous, but in- 

 differently preserved, viz. Monograptus gregarius, M. tenuis (Port- 

 lock), and Dawsonia campanulata (Nich.). 



The stream occupies a similar position with respect to the under- 

 lying rocks as far as the foot of the Spa Score, and affords at least 

 two additional sections of the higher zones of the Moffat Shales. 



