2 MR. L. RICHARDSON ON THE RH^TIC OP [Feb. I9II, 



I. Introduction. 



In Somerset the Rhaetic rocks have a greater superficial extent than 

 in any other county. It was anticipated that their investigation 

 would prove of exceptional interest for several reasons : notably 

 because — 



(a) It has long been known that the Series attains an unusual thickness in 

 places in this county, while it also frequently assumes a littoral facies 

 when in contact with the Palaeozoic rocks that formed land-areas 

 during its time of deposition ; 



(5) The discovery in the top-portion of the ' Grey Marls ' of Watchet of 

 Microlestes teeth, associated with characteristic Rhaetic fossils, has long 

 been an important fact in the discussion as to whether the ' Grey and 

 Tea-green Marls ' (auctt.) should be classed with the Rhaetic or with the 

 Keuper ; 



(c) The Pteria-contorta, Shales, with their associated hard beds, have yielded 



a rich assemblage of in-vertebrate fossils ; and 



(d) The White Lias is excellently developed, and its relationship to the 



Lower Lias can be studied in many sections. 



In order, however, that students of the Series should have at 

 their disposal easily-comprehended facts concerning the detailed 

 stratigraphy, as also a knowledge of the position of certain strata 

 bearing well-known names, and the horizons whence many of the 

 well-preserved specimens now housed in our museums came, much 

 detailed work was necessary. The main points that required to be 

 settled were : — - 



(a) The stratigraphical position of the marls that had yielded the Microlestes 

 teeth and other characteristic Rhaetic fossils ; 



(/3) The true position of the ' Wedmore Stone,' Moore's ' Flinty Bed,' and 

 the Pleurophorus Bed ; 



(y) Whether the ' Upper Rhaetic ' as developed, for example, at Garden 

 Cliff, near Gloucester, is represented in Somerset ; and 



(S) With which formation the White Lias proper is best classed — with the 

 Rhaetic, or with the Lias. 



In addition to the record of a number of new or imperfectly 

 described sections, my researches have resulted in showing that : — 



(a) The Microlestes Marls correspond to the Sully Beds of the Glamorgan 

 coast, and are well-developed in West Somerset ; 



(j3) The Pteria-contorta Shales are of greater thickness than has generally 

 been supposed, and contain, especially below the Pleurophorus Bed (13) 

 and the main Bone-Bed (that is, Bed 15), some highly fossiliferous 

 bands ; that the ' Wedmore Stone ' occurs well below the equivalent 

 stratum to this main Bone-Bed (Bed 15); and that Moore's 'Flinty 

 Bed ' of Beer Crowcombe (or ' Crocombe ' as it is now spelt) is most 

 likely on the horizon of the Pleurophorus Bed (13) ; 



(7) The ' Upper Rhaetic,' as defined in my previous papers, is as persistent 

 as usual, if not quite so thick ; 



(d) The White Lias proper is of considerable thickness in places, is in its 

 characteristic development of comparatively restricted geographical 

 extent, and is best classed with the Rhaetic ; and 



