SILURIAN SYSTKM 155 



This arrangement of the Silurian succession into Valentian, 

 Salopian, and Downtonian has been adopted by many English 

 writers, but it may be doubted whether it is any real improvement 

 on that hitherto employed and still used by the Geological Survey. 

 It is quite as important to recognise that the central mass of beds 

 can be divided into two groups by the different types of graptolites 

 as it is to admit that they are united by a certain community of 

 species. Thus the Wenlock Beds are characterised by species of 

 (Jijrtiiiiraptns and by the Monograptus priodon type, and the higher 

 beds by species of the M. colonus type. Thus the whole i 

 can be divided into four much more equal groups or series on a 

 palsjeontological basis, and it is probable that this will form the 

 classification of the future. Meantime the several arrangements 

 are shown in the following table : 



L -j a f Downton Beds 

 Downtoman-Eurvptend Series | Upper Ludlow Beds [ Ludlow 



, . f Aymestry limestone j Series. 

 ,M. Colonus Series \ L ^ yer L * d]ow ghale J 



Salopian J M. priodon and ( Wenlock limestone 1 wl,.b 



I Cyrtograptus ^ Wenlock shales 

 I Series (Woolhope limestone J " 



fTarannon shale ^ T i _j-_ 

 Valentian Rastrites and J L Llandover y 



J 



In dealing with the stratigraphy of the system, I shall use the 

 new grouping as affording convenient divisions in the description 

 of the beds*, but shall indicate the correlation of the priodon and 

 colonus series in different areas, so that the student can, if he 

 chooses, adopt the alternative arrangement of Llandovery, Wenlock, 

 and Ludlow Series. 



B. LIFE OP THE PERIOD 



The following is a brief synopsis of the principal genera which 

 are found in the Silurian System : 



Plantse. It is in this system that the earliest remains of plants 

 have been found. Some of these remains are doubtful, but tlu-iv 

 is general agreement with regard to Nematophycus and Pachytheca. 

 The former occurs in the form of silicified stems and have been 

 found in Wenlock Beds. Pachytheca is a name given to small 

 spherical bodies which may be seeds. 



Hydrozoa. Graptolites are abundant in the lower beds, but 

 gradually decrease in numbers and die out in the upper beds. 

 The family Monograptidae, including the genera Motwgraptut, 

 Cyrtograptns, and Rastrites, appears to be confined to the Silurian, 



