42 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [ June 3, 



1835*, and afterwards in the ' Silurian System' itself, that the so- 

 called " Llandeilo flags" (those in the very environs of Llandeilo) 

 consist of various bands of limestone interstratified with sands and 

 shales, which in reference to the upper strata exhibit in that tract 

 only a very thin course of sandstone, representing the great Caradoc 

 band of Shropshire. That which is thick in one place thins out in 

 another. In one tract, shale, flagstone and calcareous courses 

 prevail ; in another, sandstone and sandy limestone ; in a third, 

 shale and slaty rocks ; but all these, I contend, are characterized by 

 the same group of fossils, viz. Asaphus tyrannus, A. JBuchiiy 

 Trinuclei of various species, Illmnus crassicauda ?, and above all 

 by the simple-plaited Orthidae, Orthis Actonice, O.fiahellulum^ O. 

 lata^ &c., of which O. {callactis) calligramma is the typef. In 

 short, the protozoic group, as subsequently worked out, has now 

 been proved to be nothing more than Lower Silurian %. 



The list given by Professor Sedgwick of fossils common to the 

 Wenlock and Llandeilo rocks in those tracts where no intervening 

 sandstone occurs, may perhaps lead inexperienced observers to 

 think that there is no longer a true distinction between these 

 formations ; but in the localities to which he appeals, few of the 

 Lower Silurian fossils, except such as are common to great por- 

 tions of the Silurian system, and have a very wide geographical 

 distribution (such as Calymene Blumenbachii, Leptcena depressa and 

 L. euglypha, «Src.), run up into the Wenlock limestone, whilst the 

 presence in that rock of a single species of those simple-plaited 

 Orthidae which appear in such millions of individuals in the Lower 

 Silurian, is, when it does occur, a very rare phssnomenouj either in 

 the British Isles or Scandinavia§. 



If we confine our reasoning to Britain alone, and take the com-» 

 mon and characteristic typical shells, there can I think be no better 

 proof that the Lower Silurian is well separated from the Upper, 



* Phil, Mag. June, 1835. 



t As other explorers will doubtless visit the very instructive tract of Corndon 

 and Shelve to the south of Shrewsbury, which I have described in some detail 

 (with all its igneous rocks, both contemporaneous and intrusive), they will there 

 see a very instructive epitome of what I consider to be a group analogous to many 

 tracts in Cambria, or North Wales. The great mass of the Lower Silurian rocks, 

 as shown by my sections (Sil. Syst. pi. 32. figs. 1 and 2), terminate downwards in the 

 copious sandstones and quartz rocks of the Stiper stones, and upwards in flagstones 

 containing Asaphus Buchii, which in their turn are surmounted by Wenlock shale ; 

 thus presenting, though on a much clearer and grander scale, the same succession 

 as at Builth. Here therefore if" Llandeilo flags " are to be alone recognised by their 

 containing Asaphus Buchii, they may, according to my own published sections, 

 be said to overlie Caradoc sandstone. But as mineral constants are unknown, 

 so at Llandeilo the alternating lower fossiliferous series of sandstones and lime- 

 stones is separated from the upper group by quartz or sandstone. It is to the 

 ** Lower Silurian " fossil types only that I appeal. 



t We have yet to be made acquainted with those forms announced by Professor 

 Sedgwick to distinguish his Protozoic Cambrian from my Protozoic Lower Silu- 

 rian (as published). 



§ Amidst a profusion of Upper Silurian shells, one simple-plaited Orthis (a 

 variety of Orthis calligramma) was found by myself in the lowest shale (Wenlock) 

 of Gothland, and of this species one or two indi\dduals only were detected. 



