Reviews — The Surve// Memoir on the Scottish Uplands. 477 



litbological rock-bands is that of a geologically ascending succession 

 of Silurian rock formations, embracing some eight distinct rock 

 groups, all of which (except the highest) form, as shown by their 

 collective graptolite fauna, a series of ascending stages in one general 

 Upper Llandeilo series. The lowest formation of this Llandeilo series 

 dips visibly oif the great longitudinal anticlinal axis of Hawick 

 and Dumfries, and its highest subsides below the core of the great 

 Leadhills' synclinal form. Collectively, this series must be of 

 enormous thickness, but is remarkably monotonous both as regards 

 mineralogical character and fossils. And, judging from all these 

 conclusions, it would appear that in Upper Llandeilo times this was 

 a region of shallow and muddy seas, in which there was for ages 

 little or no change in the physical conditions and little or no 

 progress in the evolution of organic life. 



If, on the contrary, we follow the newer and more complicated 

 zonal methods of mapping the Uplands — in other words, by detailed 

 criteria primarily palseontological — we soon find ourselves shut up 

 to the conclusion that the graptolite species, instead of being, as 

 was formerly believed, one of the most untrustworthy of geological 

 witnesses, is in reality one upon whose testimony the stratigraphist 

 can implicitly rety. We gather that the conclusions drawn from 

 the evidences it affords accord perfectly with the detailed litbological 

 evidences and the like ; they allow us so to explain and correlate the 

 ascertainable petrological and palaeontological phenomena from end 

 to end of the Upland region, that they afford us a clue to the true 

 arrangement of its strata and the key to its structure. 



We learn that the grand geographical rock-bands of the Uplands 

 are not the outcrops of an ascending series of distinct rock-groups, 

 but answer, broadly speaking, to those of the different litbological 

 variations of strata laid down in the same geological pei-iod, as they 

 slowly change in mineral character and in thickness when followed 

 from the ancient Silui"ian shore-lines towards the depths of the seas. 



We find that not only is the petrological distinctness of the various 

 rock-bands no proof of their individuality, but that the relative 

 thickness of such Upland formations as we can individualize has 

 no relation to their relative geological importance. The insignifi- 

 cant group of the Moffat Shales, only some 300 feet in thickness, 

 formerly classed as a mere subdivision of the Upland Llandeilo, is 

 the representative in time of the three great Welsh formations, the 

 Upper Llandeilo, the Bala, and the Llandovery, and, like them, 

 is marked by three distinct faunas. On the other hand, the Upland 

 Gala or Queensberry formation, 25 miles in breadth and many 

 thousands of feet in thickness, must be looked upon merely as 

 a magnified representative of the band of Tarannon Shale, which in 

 Wales is only about 1,000 feet in thickness, and marks the transitional 

 period of time between the Llandovery and the Wenlock. 



But not only are many of the stratigraphical generalizations drawn 

 from broad regional views of the strata of the Upland region thus 

 superseded by others due to the application of the zonal methods, 

 but also many of the tectonic conclusions drawn from broad views 



