C. LAP WORTH ON THE MOFFAT SERIES. 301 



several distinct horizons in the great greywacke formation, or on the 

 more popular hypothesis that they belonged to a single and persistent 

 sheet, both underlain and overlain by arenaceous beds. They now 

 find their common explanation in the simple circumstance that the 

 dark shales reach the surface of the country along the chief anti- 

 clinal lines, the width of the several exposures being dependent 

 merely upon the varying elevation of the crown of the arch. 



The extraordinary diversity apparent in the various groups of 

 species yielded by the strata of the same band in the different expo- 

 sures along its course, and the peculiar localization of some of its 

 most distinctive fossil forms, are quite as easily explained by our 

 discovery of the rigid restriction of the Graptolitic species to definite 

 zones. Not only do the earliest zones of each band make their ap- 

 pearance as a rule only in the widest exposures, but its apparent 

 fauna in any single locality is necessarily that of the collective faunas 

 of the special zones, which the concurrent accidents of plication, 

 metamorphism, and denudation have there left accessible to the 

 investigator. Lastly, we have no longer any room for astonishment 

 at the remarkable similarity in lithological characters, the great 

 lateral extension, persistent north-west inclination, and apparently 

 gigantic thickness of the interminable greywackes of the Moffat 

 district. We are now satisfied that they form in reality a single 

 group of beds of no great vertical dimensions, the same strata 

 being repeated again and again in rapid and partially inverted 

 undulations. 



B. Subdivisions, Lithology, and Paleontology oe the Mofeat 



Series. 



§ I. The Glenkiln Shales. 



From the foregoing description of the several sections of the 

 Moffat Series within the limits of the present district it is evident 

 that we are far from possessing an equal knowledge of the rocks and 

 fossils of each of its three chief divisions. The general lithological 

 and palseontological characteristics of the Birkhill division may be 

 gathered from at least fifty different sections. The total number of 

 appearances of the Hartfell Shales cannot exceed ten or twelve. Of 

 the lowest division, or Glenkiln Shales, there are only five exposures, 

 and in these the strata exhibited are mere fragments of the succes- 

 sion, cut off as a rule by slips or faults from all their original asso- 

 ciates. 



Naturally, therefore, our present knowledge of the Glenkiln divi- 

 sion is far inferior in amount and accuracy to our knowledge of the 

 overlying divisions. Prom the circumstances of the case we are un- 

 acquainted with any thing that ought properly to be regarded as the 

 base of the division in this district. We are, however, able to deter- 

 mine with precision its superior limit, and at the same time to prove 

 that in vertical extent it equals the succeeding divisions, and is 

 characterized by a fauna correspondingly distinct. 



