Maury — Calcium Carbonate Concretionary Growth. 369 



Art. XX XL — A Calcium Carbonate Concretionary Growth 

 in Cape Province ; by C. J. Mauey. 



The origin of the lime deposit owned and quarried by Mr. 

 Stephen Dahse, near Hermon, forty miles north of Cape Town 

 has been accounted a baffling puzzle of Cape geology. The lime 

 lies in the Malmesbury shales and forms a small dome occupying 

 the center of a low hillock rising from the surrounding level 

 veldt. A few calcareous bowlders chanced to be found on the 

 top of the hill and led to the exploitation of the dome beneath. 

 Near the surface the lime rock is weathered into odd, fantastic 

 shapes, but deeper down it becomes hard, crystalline, and of 

 great purity. It may be noted thai the last characteristic alone 

 marks the deposit as rather unique, for pure lime free from 

 magnesia or silica is a valuable rarity in South Africa. On 

 chemical analysis the Hermon deposit yields 99 per cent CaCOg. 

 The rock is so handsome that efforts were made to cut it into 

 ornamental slabs, but its tendency to break along the rhombo- 

 hedral cleavages could not be overcome; hence it is roasted in 

 kilns and serves for plastering and whitewashing the tine old 

 Cape Dutch houses. 



At the time of my visit the lime quarry had been exploited 

 to a depth of about seventy-five feet and a diameter of about 

 two hundred. The circular form of the calcium mass was 

 completely revealed as it lay encircled by the Malmesbury 

 shales. These are typically laminated, much fractured, and 

 steeply inclined. They constitute the oldest rocks of the Cape 

 region. In the vicinity of their contact with the lime dome 

 they are greatly decayed and disintegrated into a very unc- 

 tuous clay. The plane of contact was very markedly slicken- 

 sided. 



It is my belief that this singular formation is a concretionary 

 growth parallel in origin to the salt domes of Louisiana. Pro- 

 fessor G. D. Harris,^ then State Geologist of Louisiana, origi- 

 nated the ingenious theory that these domes or '\salt islands" 

 are huge concretions formed by crystal growth from material 

 in aqueous solution. 



This theory was rendered still more convincing by the exper- 

 iments of Mr. Stephen Taber f and those of Miss Long,:}: both 

 series demonstrating the force that growing crystals exert in 

 overcoming external resistance. Mr. Taber was ied to conclude 

 that the force thus arising may have played an important role 

 in the formation of veins and concretions. 



* Louisiana Geological Survey, Bulletin No. 5: also Bulletin No. 7, pp. 

 75-88, 1908. 



f This Journal (4), vol. xli, pp. 532-556, June 1916. 

 ilbid., vol. xliii, pp. 289-293, April 1917. 



