492 Prof. Garwood — Calcareous Algae. 



therefore, extends the vertical range of this widely distributed genus 

 down to the oldest Palaeozoic rocks. It is interesting to note that 

 the rocks in which the Solenopora occurs on the Antarctic Continent 

 contain a development of pisolite and oolite, and that this is also the 

 case in the Australian equivalents. In 1887 and again in 1891 

 Bornemann described and figured species of Siphonetna and Confervites^ 

 from the ArehcBocyatlms limestones of Sardinia. As regards the former 

 genus, it was shown by Dr. Hinde" to be congeneric with Girvanella 

 (Xich. & Eth.). It is of interest, however, to note that Borneraann 

 describes this form as a calcareous alga, and compares it with existing 

 subaerial Algse growing on the surface of limestone rocks in Switzer- 

 land. The latter genus is stated by Seward to be possibly " a 

 Cambrian alga, but the figures and descriptions do not afford by 

 any means convincing evidence" {Fossil Plants, vol. i, p. 178). 



More recently, in 1904, Dr. T. Lorenz ^ has described remains of 

 Siphonece from the Cambrian rocks of Tschang-duang in Northern 

 China, for which he erects two new genera, Ascosoma and Mitscher- 

 lichia, placing them in a new family, the Ascosomacese. These Algae 

 build important beds of limestone, the individuals often attaining 

 a length of 4cm. and a thickness of l'5cm. In 1907 Bailey Willis* 

 reported Girvanella associated with oolites in the lowest Cambrian 

 Man-t'o Beds in China. It is probable, therefore, that as our knowledge 

 of these rocks is extended. Calcareous Algae will be found to play an 

 important part in the Cambrian limestones of the Asiatic Continent 

 and Australia.* 



Ordoviciak. 



In the Ordovician rocks the remains of Calcareous Algae become 

 much more abundant ; they are very widely distributed and for the 

 first time they become important rock-builders. In Britain the chief 

 genera met with are Girvanella and Solenopora. These two organisms 

 occur abundantly in the Scottish Ordovician rocks of the Girvan area, 

 where they appear to have contributed largely to the limestones of 

 the Barr Series in Llandeilo-Caradoc times. 



As already mentioned, Girvanella prohlematica was originally 

 described by the late Professor Nicholson and Mr. K,. Etheridge, jun., 

 from the Craighead Limestone at Tramitchell, where it occurs in 

 great numbers. The ofiicers of the Geological Survey also report it 

 from the Stinchar Limestone of Benan Hill.® It occurs in the form 

 of small rounded or irregular nodules, varying in diameter from less 

 than a millimetre to more than a centimetre, many of the nodules 

 showing marked concentric structure. In Benan Burn, where these 

 beds are admirably exposed, the Girvanella nodules appear con- 

 spicuously on the weathered surfaces, being so abundant as to 

 constitute thick layers of limestone. 



1 Nova Acta Cees. Leop. Car., 1887 and 1891. 

 ° Hinde, Geol. Mag., Dec. Ill, Vol. IV, p. 226, 1887. 

 5 Centralb. f. Min., 1904, p. 193. 

 * Research in China, 1907. 



5 Chapman, Proc. Key. Soc. Vict., 1911, p. 308. 



•* The Silurian Rocks of Britain, vol. i, Scotland (Mem. Geol. Surv. U.K.), 

 pp. 487, 494, 496, 500. 



