180 Mr. H. B. Brady on Saccammina Carteri, 



It seemed desirable to compare the Elf hills rock with other 

 spheroidal and concretionary limestones of palajozoic age ; and 

 for the means of doing so I am indebted to the kindness of 

 Mr. Etheridge, the palaeontologist to the Geological Sm-vey, 

 who has furnished me with a number of specimens of such 

 limestones, some of them Carboniferous, others from the 

 Wenlock and Bala beds. In each of the specimens there is 

 somQ j^fimd-facie resemblance to the Elf hills rock ; and in one 

 or two the similarity is so striking that the naked eye is hardly 

 sufHcient to discern the wide difference that really exists be- 

 tween them. By means of transparent sections and a good 

 microscope, the true structure is readily made out ; and in all 

 the specimens sent by Mr, Etheridge it is essentially the same. 

 They are composed of laminated spheres of carbonate of lime, 

 formed by the common process of spherical coalescence ; and 

 that their physical peculiarities are in no way due to organic 

 remains may be asserted with certainty in every instance. 



CJtemical. — Although the Elf hills limestone is as compact 

 as many varieties of marble, considerable difficulty is expe- 

 rienced in obtaining a polished surface by grinding, owing to 

 the different degrees of hardness of its constituents. The ma- 

 trix is usually softer than the fossils imbedded in it ; and fre- 

 quently the infiltrated matter which occupies the interior of 

 the spheres is harder than its investing shell. This was found 

 to be due to the presence of silica unequally distrilmted. The 

 matrix appears to be free from silica ; but a number of un- 

 broken spheres washed quite clean were submitted to analysis 

 by my friend Mr. A. Freire-Marreco, and found to contain as 

 follows : — 



Silica 44-66 



Iron and alumina 4-86 



Carbonate of lime 48-33 



Loss 2-15 



100 parts. 



The casts and crystalline tufts which occupy the interior of 

 the spheres were subsequently examined, and found to yield 

 92 per cent, of silica. The analysis was made from forty or 

 fifty of the "cores" taken at random; but the percentage 

 named can scarcely be said to represent accurately the propor- 

 tion of siliceous and calcareous infiltration. The siliceous 

 casts are usually amorphous (colloid), and completely fill the 

 cavities ; and if the interior be smooth, they enucleate them- 

 selves as solid spherical masses on the fracture of the arena- 

 ceous investment. The crystalline cores, on the other hand, 



