8 Proceedings of the Royal Physical Society. 



tion. He has further shown that the IMoffat black shale 

 series, varying from 300 to 500 feet in thickness, is divisible 

 into several zones, each containing a distinct suite of grapto- 

 lites. By means of these zones he has demonstrated that 

 many of the anticlinal folds are inverted, so that what 

 appears at first sight to be an ascending series is in reality 

 deceptive. In the prosecution of the Geological Survey of 

 that region I have had exceptional opportunities for examin- 

 ing the sections described by Professor Lapworth, and it 

 gives me pleasure to express my appreciation of the accuracy 

 of his observations and of the value of his researches. But 

 when we pass beyond the limits of the Moffat district the 

 classification of the black shales undergoes important modifi- 

 cations. It sometimes happens that certain graptolites, which, 

 in the Moffat area, are confined to separate horizons, are 

 found side by side in the same band. For example, near 

 Castle-Douglas some of the fossils characteristic of the Glen- 

 kiln group in the Moffat region are associated with grapto- 

 lites belonging to the horizon of the Hartfell beds, while in 

 another direction the members of the Birkhill group disap- 

 pear altogether, and are represented only by barren grey- 

 wackes and shales. Some forms, which he assumes to have 

 become extinct during the deposition of the Moffat group, 

 reappear in the greywackes overlying the black shales. The 

 assumption that the disappearance of a species from a par- 

 ticular horizon implies its extinction, when it may be due 

 merely to a change of conditions causing temporary migra- 

 tion, may be adduced as an instance of unsafe reasoning from 

 purely negative evidence. Such an inference is never safe, 

 and is apt to lead to erroneous conclusions. Professor Clay- 

 pole, of the Pennsylvanian Geological Survey, has recently 

 published parallel instances from the Devonian strata of the 

 United States, where spirifers occur in the same slab which 

 were supposed to characterise separate zones of that forma- 

 tion. He also shows that Halycites catenularia, which was 

 believed to be a characteristic fossil of the Niagara Limestone, 

 and to have become extinct at the close of the deposition of 

 this zone, reappears in other limestones occupying a higher 

 horizon. The case of Bictyonema, which was supposed to 



