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XVI. — Note on an Algal Limestone from Angola. By Mrs M. F. Romanes, late 

 Harkness Scholar, Newnham College, Cambridge. Communicated by Professor 

 J. W. Gregory, F.R.S. (With One Plate.) 



(MS. received June 2, 1915. Read June 28, 1915. Issued separately April 25, 1916.) 



The specimens here described were collected by Professor J. W. Gregory in Angola, 

 and belong to the series described in the paper by him on the geology of that country. 



The horizon of the beds from which the limestone was obtained is Albian. 



The specimens are of a yellowish pisolitic limestone. In hand specimens the 

 weathered surface has a warty appearance — the diameter of each knob being about 

 1 cm. A fractured surface shows circular forms varying in size and having decided 

 concentric markings. These concentric markings are, however, less evident in 

 microscopic sections, and this is probably due to the effects of preservation. A 

 similar case has been described by Mr H. Yabe, # where the genus Metasolenopora is 

 affected in this way. These circular forms make up the greater part of the rock, and 

 are cemented together by a fine calcareous paste in which are fragments of shell and 

 quartz grains, with occasional grains of other minerals, such as felspar and muscovite. 

 The specimens of limestone examined varied to some extent in texture, some being 

 more compact, and even slightly siliceous, but the contained organisms are present 

 in similar amounts regardless of the texture of the rock. 



With care, small roughly circular nodules can be extracted from the looser 

 varieties of the limestone. 



In thin section, under the microscope they are seen to consist of algal growth 

 round a nucleus. The substance of the nucleus varies, and may be the plate of an 

 echinoid, or more commonly is the fragment of a spine. In some cases a group of 

 quartz grains serves as a centre round which growth has taken place. Instances of 

 this have been noted by Mr Wethered t among the pisolites of the Jurassic. Some- 

 times also small bodies, cf. calcispher8e,| help to form a nucleus. Some also occur 

 scattered throughout the matrix of the rock. Their possible presence in this lime- 

 stone is interesting in view of the fact that the association of calcispherse with 

 abundant plant remains has already been noted. § 



Two different genera of algae have been determined making up the pisolitic nodules. 

 The genus present in the greatest abundance has been identified as Girvanella Nich.|| 



* H. Yabe, Sci. Rep. Imp. Tohoku Univ. Sendai, Japan, vol. i, No. 1, 1912, p. 2. 



t E. Wethered, Geol. Mag., dec. 3, vol. vi, 1889, pp. 196-200. 



J E. J. Garwood, Q.J.G.S., vol. lxviii, 1912, p. 580, pi. xlvii, "Carboniferous Succession in N.W. England." 



§ R. C. M'Le^n, Proc. Camb. Phil. Soc, vol. xvi, pi. vi, 1912, p. 512, "Group of Rhizopods from Carboniferous 

 Period." 



|| A. Nicholson and R. Etheridge, A Monograph of the Silurian Fossils of the Girvau District, Edin., 1878, p. 23, 

 pi. ix, fig. 24. 



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