360 SCIENCE PROGRESS. 



similar, it may be considered under the same head. It was 

 whilst working at these beds that Lapworth gave the clue 

 to the geology of the Scotch Southern Uplands (i) and 

 established that zonal method of work amongst the older 

 rocks which has led to such excellent results. The use of 

 graptolites as indices of geological horizons has since been 

 fully justified, and the consequence is that our knowledge 

 of the minute subdivisions of the lower Palaeozoic strata is 

 now far in advance of that of many rocks which are much 

 higher in the geological column. Lapworth himself ex- 

 tended his observations on the classiticatory value of the 

 graptolites to areas remote from his typical Moffat district 

 (19), and similar investigations have since been made in 

 other areas by a number of other observers, amongst 

 whom Tullberg stands prominent (18). As Lapworth 

 remarks, " in the face of these results the host of 

 proofs formerly supposed to be afforded by the abnor- 

 malities of the vertical distribution of the graptolithina 

 in favour of the doctrines of migration and colonies 

 vanish into thin air," a statement which was by no 

 means generally accepted at the time it was written 

 (in 1879) ; but so rapid has been the increase of our 

 knowledge since then that the same writer was enabled 

 to observe in 1889 that the zonal method has "quietly 

 entered upon what may be regarded as the accepted or 

 orthodox stage ". 



Before leaving the Silurian rocks, reference must be 

 made to our present knowledge of the line of demarcation 

 between the rocks of the Silurian and Devonian systems. 

 Unfortunately, in Devonshire, where the normal marine 

 sediments of Devonian age are developed, there is no pas- 

 sage downwards into the lower Palaeozoic strata, whilst in the 

 Welsh borderland, where a passage occurs, the Devonian 

 beds are of the sparsely fossiliferous old red sandstone type. 

 In other areas, the transitional beds are better represented 

 and it is possible to draw a satisfactory line at the top of the 

 Silurian system. The occurrence of Eurypterus Fischeri 

 at the summit of the Silurian system in Norway, Gothland, 

 Russia and North America has been commented on by 



