Vol.62.] HIGHEST SILURIAN ROCKS OF THE LUDLOW DISTRICT. 197 



II. Classification. 



In the Ludlow-Downton district there exists an interesting series 

 of rocks, limited by the Ayraestry Limestone at their base and the 

 Old Red Sandstone at their summit, and it is with these that 

 the present paper deals. Lithologically, they present a varied 

 series of sediments, ranging from limestones on the one hand, 

 through calcareous flagstones and shales, to shallow- water sand- 

 stones on the other ; and these lithological changes are associated 

 with, certain changes in the fauna. 



Palseontologically, these rocks are characterized by the presence 

 of Eurypteridse, which, although rare in the lower beds, gradually 

 increase in importance until they attain their maximum development 

 in the beds immediately underlying the Old lied Sandstone. The 

 rich brachiopod-fauna characteristic of the lower beds dwindles and 

 almost dies out with the approach of shallow-water conditions, 

 although the molluscs are somewhat more persistent. 



The recurrence throughout of conditions tending to the formation 

 of ' Bone-Beds ' is also worthy of note, such conditions having 

 prevailed at four distinct times at least during the deposition of the 

 rocks with which we are concerned. 



As the result of detailed stratigraphical and paleeontological work, 

 we believe that we are able to show that these highest Silurian rocks 

 are capable of a certain degree of subdivision, both on lithological 

 as well as on palseontological grounds. Future work will decide 

 whether or not these divisions have a more than local value. 



The subdivisions that we would suggest are tabulated on p. 198. 



The Aymestry Group takes its name from the well-known 

 locality of Aymestry, 8 miles south-west of Ludlow; its lower 

 member, the massive GoncJiidium-Limestone is, as a rule, full 

 of Conchidium (Pentamerus) Knightii, and is thus easily recognized. 

 At Aymestry this ■ limestone is only 75 feet thick ; it thickens, 

 however, in a northerly direction, being about 100 feet thick near 

 Ludlow, and as much as 250 feet at Mocktree. The Mock tree 

 or Dayi a -Shales, which overlie it everywhere, vary according 

 to the thickness of the limestone-bands that they contain ; these 

 thicken also to the north, and thus, while only 40 feet thick near 

 Ludlow, have a thickness of 150 feet on Mocktree Hill, whence 

 they derive their name. They are invariably crowded with Dayia 

 navicula. These Mocktree Shales seem, as a rule, to have been 

 regarded as forming part of the Upper Ludlow Group, but we are 

 convinced that the brachiopod-fauna is more closely allied to that of 

 the Aymestry Limestone than to that of the higher calcareous beds ; 

 and, in addition, Monograjptus leintwardinensis, which certainly 

 occurs in the Aymestry Limestone, is also found in the highest beds 

 of the Mocktree Shales. Therefore we place the Mocktree Shales 

 in the Aymestry Group. 



The Upper Ludlow Group is also capable of a twofold 

 subdivision into the Lower and Upper Whitcliffe Flags. 

 These derive their name from the lofty cliff which forms the right 



