C. LAPWOBTH ON THE MOFFAT SEMES. 249 



their apparent order of superposition in any single section, however 

 perfect. 



(a) Subdivisions, Characteristics, and Inter-relationships of the Strata 

 exhibited in the typical section of the Main Cliff. 



The only locality in the glen where the junction of the grey- 

 wackes with the dark shales and mudstones is distinctly visible for 

 some distance is in the floor and sides of the lateral gorge imme- 

 diately below the falls (Plate XII. Sect. I.). Here the sequence 

 from the greywackes for at least 100 feet into the very heart of the 

 Moffat beds is seen to be unbroken ; and the evidences afforded by 

 the bed of the stream can be easily checked and supplemented at 

 every stage of our inquiry by a comparison with the frequent 

 exposures of the same strata in the South Cliff, in the floor and walls 

 of the steep " corrie" to the north-west, or along the bottom of the 

 little rill that trickles down its centre. 



Commencing with the greywackes of the falls, which, dipping to 

 the south-east at a very high angle, visibly pass below all the mud- 

 stones and shales of the glen, and reading off the characters of the 

 consecutive beds in what thus appears to be the ascending order, we 

 note the following succession (Vertical Sect. fig. 1, p. 250) : — 



(I.) First or BirJchill Division. 



(i.) First Grey-Shale Group. ft 



1. Grey and greenish shales and flaggy beds 15 



2. Grey shales containing four black bands, from 2 to 4 inches in 



thickness, and occasional lamin£e of soft shaly clay of a pure 

 white or cream-colour. Graptolites are numerous in the black 

 seams. The most characteristic forms are Bastrites maximus 

 (Carr.), Monograptus Haiti (Barr.) 10 



3. Grey shales with white-clay bands, barren of fossils 15 



4. A peculiar assemblage of black, white, and greyish-brown shales 



and mudstones. Some of the dark mudstones are soft and 

 highly pyritous. A few thin seams are sandy and harsh to the 

 touch. The latter are sometimes cut up superficially into lozenge- 

 shaped reticulations by a series of shallow grooves, somewhat 

 resembling sun-cracks, but always perfectly straight throughout. 10 



Fossils are abundant in the darker beds. The most prevalent 

 forms are Monograptus Sedgwicki (Portlock)=ilf. spinigerus 

 (Nich.) and Monograptus Hisingeri (Oarruthers). 



5. Grey mudstones, with seams of white clay 15 



Near the middle of this band occurs a thin seam of black shale, 

 which is well shown at the head of the Corrie. It is about 

 6 inches in thickness, and is charged with Monograptus Clingani 

 (Carr.) and Bastrites hybridus (L.). Immediately below the 

 fossil-bed in this section is a curious seam of dark grey mud- 

 stone, covered with the trains of Annelides, and full of small 

 nodular concretions. 



6. Zone of highly pyritous black shale, disintegrating into small flaky 



fragments, which are coated with a rusty film of iron oxide, or 

 flowers of alum. It is crowded with poorly preserved speci- 

 mens of Monograptus lobiferus (M'Coy), Diplograptus cometa 



(Gein.) 8 



This is covered by 4 feet of barren grey shale, above which 

 follow the first beds of the succeeding black-shale group. 

 Q.J.G. S. No. 134. s 



