Distribution of the Rhabdophora. 47 



cepted) the physical and palasontological changes at the base 

 of the equivalents of the Lower Llandovery are so marked 

 and of such systematic importance, that American geologists 

 have universally drawn the boundary line between their Lower 

 and Upper Silurian systems along this horizon. 



Adopting, then, this line as the base of the true Silurian, we 

 have next to determine the most natural limits of the compo- 

 nent formations of that system. Here we have to bear in 

 mind that in the typical area of Shropshire a few feet only of 

 the lowest of the Silurian formations are visible, and even 

 these were not separated by Murchison from the underlying 

 Bala rocks until he was compelled to distinguish them by the 

 discoveries of Sedgwick and M'Coy. And it is now daily 

 becoming more clearly evident that in the same typical area 

 there is, in reality, but a very feeble and degenerate represen- 

 tative of the highest formation of the Silurian rocks of other 

 countries — the wonderfully prolific etages F and G of Bohe- 

 mia, and the great Helderberg series of North America. 



Of the first of the three grand formations into which the 

 Silurian may most naturally be divided, all that is exposed in 

 the typical area of Shropshire are the thin zones of the 

 Henley conglomerate and the Pentamerus-Liimestone, which lie 

 between the summit of the Bala and the base of the Wenlock 

 shale. In many parts of Wales, however, as is well known, 

 we find three distinct groups of strata in this position, separated 

 from each other by fairly marked unconformabilities. It has 

 generally been the habit to call the first of these subforma- 

 tions by the title of the Lower Llandovery, the second May- 

 hill or Upper Llandovery, and the third the Tarannon shale. 

 The first is usually believed to be most intimately allied in its 

 palffiontological characters to the Bala formation, and the 

 last to be hardly separable from the Wenlock shale. My own 

 researches impel me to the conclusion that these three sub- 

 formations are far more closely allied to each other than they 

 are to the beds above or below, and that they should be consi- 

 dered as the three consecutive members of a single formation. 

 In the south of Scotland ( Valentia) these three subformations 

 are recognizable, superposed in conformable sequence, with 

 clear relations to the Bala below and to the Wenlock above, 

 and unitedly covering an area of several thousands of square 

 miles. Until geologists are willing to include the Tarannon 

 in the Llandovery it will therefore be best to speak of this 

 great Scottish formation and its equivalents as the Valentian 

 formation, its three divisions, Lower, Middle, and Upper, 

 representing respectively the Lower Llandovery, Upper Llan- 

 dovery, and Tarannon of Wales and Siluria. 



