22 PEOF. E. J. GARWOOD AND MISS E. GOODYEA.E [vol. lxxiv, 



present day) of less than a mile, appears to be only explicable on 

 the supposition that the beds at the ' Sandbanks ' were deposited 

 in direct connexion with the open Woolhope sea ; while the reef- 

 like development at Nash Scar and Old Radnor was accumulated 

 round the islands of a slowly-subsiding archipelago, in lagoons 

 sheltered from the open sea and free from the detrital deposits of 

 rivers. 



This reef-facies of the Woolhope Limestone at Old Radnor and 

 Nash Scar, with its well-developed bryozoan growths and abundant 

 calcareous alga?, is most nearly paralleled by the algal and reef 

 developments of Wenlock and Lower Ludlow age in Southern 

 Gotland, as described by Dr. H. Munthe. 1 



Here two horizons occur, characterized by a profuse algal develop- 

 ment, notably Solenopora gotlandica Bothpletz and Spliarocodium 

 gotlandicum Rothpletz : the Lower or Splicer ocodium Marl, 

 and the Upper or Splicerocodium Limestone. The former, which 

 stretches across the southern part of the island from Burgsviken 

 to Bandelundaviken, and occurs also in Tofta, is correlated by 

 Dr. Munthe with the Wenlock Limestone ; while the sandstone, 

 oolite, and Upper Splicerocodium Limestone which overlie it, are 

 considered to represent the Lower Ludlow. Of these beds, as a 

 whole, Dr. Munthe remarks that they 



' are a closely -related series, both palaeontologically and also, in part, litho- 

 logically. They are all shallow- water deposits, and were, in part, formed 

 upon a shore.' (Op. jam cit. p. 1415.) 



In several places reef -limestones replace the Splicerocodium Lime- 

 stone, often abruptly, and in the formation of these Stromatopora 

 and bryozoa play an important part. On the whole, then, although 

 a somewhat younger series, the Gotland beds were formed under 

 conditions very similar to those under which the algal limestone 

 of Old Radnor and Nash Scar were deposited, and the species of 

 Solenopora and Splicerocodium, so abundant in the limestone of 

 the Welsh Border, may be looked upon as the direct ancestors of 

 those which contributed so largely to the algal reefs of the Middle 

 Silurian of Southern Gotland. 



IV. Movements in the District. 



Post-Longmyndian and Pre- Siluria n. — That the Pre- 

 Cambrian rocks had undergone considerable crushing before Silurian 

 times is evident from a study of the beds in the field. The present 

 distribution of the conglomerates, their want of continuity, and the 

 restricted character of the exposures, are not easily explicable as the 

 result of the post-Silurian movements alone. The grits and grey- 

 wackes are again everywhere cracked and breceiated, even at some 

 distance from the post-Silurian faults and thrusts, and. as pre- 

 viously stated, appear to have been already in this condition when 



1 ' On the Sequence of Strata within Southern Gotland ' Geol. Foren. Stock- 

 holms Forhandl. vol. xxxii (1910) p. 1897. 



