Prof. Garwood — Calcareous Algce. 493 



Solenopora compacta, var. Feachii, which likewise forms important 

 masses of limestone, is found, like Girvanella problematica, most 

 abundantly in the Girvan area, but at a somewhat lower horizon, 

 namely, in the 'nodular limestone' and shales forming the lower 

 subdivision of the Stinchar Limestone. The horizon of the Stinchar 

 Limestone is correlated by Professor Lapworth with the Craighead 

 Limestone, and considered to represent the summit of the Llandeilo 

 or the base of the Caradoc rocks of the Shropshire district. It is of 

 interest to note that Solenopora is here accompanied at times by well- 

 marked oolitic structure, and that the same is true of the pebbles 

 with which it is associated in the conglomerate at Habbie's Howe. 



Although the marked development of Solenopora found in the 

 Stinchar Limestone ceases with the advent of the Benan conglomerate, 

 the genus appears to have survived in the Grirvan district into Upper 

 Caradoc times, for Dr. Brown describes a new species (5. litho- 

 thamnioides) from Nicholson's collection from the Ordovician 

 (? Silurian) beds at Shalloch Mill, where it is said to occur in conical 

 masses the size of a walnut. Specimens of Solenopora from Shalloch 

 Mill in Mrs. Gray's collection were obtained from Professor Lapworth's 

 Whitehouse Group. 



South of the Scottish border there is, so far as I am aware, only 

 one locality from which Calcareous Algse have been recorded in rocks 

 of Ordovician age, namely, Hoar Edge in Shropshire. Here large , 

 examples of Solenopora compacta were obtained in 1888 by Professor 

 Lapworth from the calcareous layers near the base of the Hoar Edge 

 Sandstone. The specimens were handed to Professor Nicholson, who 

 records the circumstance in his description of S. compacta in 1888.^ 

 The form occurs here at the base of the Caradoc beds, and therefore 

 at an horizon which corresponds closely to that of the Craighead 

 Limestone of Girvan. 



Professor Lapworth also informs me that he has obtained specimens 

 of Sole^iopora from a limestone in south-west Kadnorshire. As the 

 upper portion of the limestone in which it is found contains a Silurian 

 fauna, it is possible that it is here present at a higher horizon, though 

 the constancy with which it occurs elsewhere, in beds of Llandeilo- 

 Caradoc age, would seem to point to the possibility that beds of 

 Upper Ordovician age are also present in this area. In any case its 

 occurrence here is of considerable interest. 



Foreign Ordovician. 



Outside of Britain the most important development of Calcareous 

 Algae in rocks of Ordovician age is found in the Baltic Provinces. 



Solenopora occurs here in the Upper Caradoc or Borckholm Beds 

 of Schmidt's classification — where it makes up thick beds of lime- 

 stone — and it is noteworthy that this horizon is practically identical 

 with that at which S. lithothamtiioides (Brown) occurs at Shalloch Mill. 



Other specimens of Solenopora were collected by Professor jSTicholson 

 in Saak, south of Reval, from the underlying Jewe Beds, an horizon 

 which must correspond very closely to that of the Craighead Limestone 



^ Geol. Mag., Dec. Ill, Vol. V, p. 22, 1888. 



