48 Mr. C. Lapworth on the Geological 



The second natural division of the Silurian system is un- 

 doubtedly Murchison's Great Mudstone series, which includes 

 the so-called Wenlock and Lower Ludlow groups, as high as 

 the horizon of the Aymestry Limestone. In Shropshire this 

 great mudstone or Salopian formation is by far the most 

 important physical group in the Silurian. Murchison drew 

 the line of demarcation between his Wenlock and Ludlow 

 formations at the Wenlock Limestone. We suspect, how- 

 ever, that this was done less from a palfeontological than from 

 an assthetic point of view, and mainly for the sake of physical 

 symmetry. Murchison admits again and again that his Lower 

 Ludlow is " simply an upward prolongation of the Wenlock 

 shale." The natural boundary is therefore at the summit 

 of this great mudstone group, generally along the line of the 

 Aymestry Limestone, where new physical conditions set in 

 and the rocks contain a comparatively new fauna. Although 

 this improved arrangement destroys the apparent symmetry 

 of the so-called formations of Siluria, I doubt not that its 

 advantages will in time ensure its general adoption. Under 

 this scheme difficulties that have hitherto confronted us in our 

 endeavours to parallel the British and foreign strata of Silu- 

 rian age would almost wholly disappear ; the arrangement of 

 the Welsh strata would lose much of its presently acknow- 

 ledged artificiality, and approximate much more closely to the 

 order of nature all over the world. 



In Shropshire this Salopian or Mudstone formation is over- 

 lain by the sandy strata of the Upper Ludlow, the Bone-beds 

 and the Downton Sandstone. For the sake of distinction 

 these may collectively be termed the Doivntonian formation. 

 Above Llangadock these strata are almost as thick as the Wen- 

 lock and Ludlow beds united. As a rule, however, they form 

 but a very insignificant representative of the great limestones 

 F and G of Bohemia and the Helderbergs of North America, 

 the Oesel beds of Esthonia, &c. Their relation to the Dingle 

 beds of Ireland and the fossil-bearing Lower Old Red rocks 

 of Scotland it is as yet impossible to determine. 



Valentian or Llandovery Formation. 



Wales. — No Graptolites have hitherto been quoted from the 

 undisputed Llandovery strata of South Wales ; nor was I able 

 personally to detect a fragment in my hasty examination of 

 the typical localities during the summer of last year. 



I discovered Rhabdophora, however, in abundance in the 

 shales of the so-called Tarannon of the neighbourhood of Con- 

 way, North Wales. In the cliffs opposite the picturesque old 

 castle I detected 



