MR. H. B. BRADY ON SACCAMMINA CARTERI. 273 



usually softer than the fossils imbedded in it, and frequently the 

 infiltrated matter which occupies the interior of the spheres is 

 harder than the investing shell. This was found to be due to the 

 presence of silica unequally distributed. The matrix appears to 

 be free from silica ; but a number of unbroken 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 

 ninety-two per cent of silica. The analysis was made from 

 forty or fifty of the " cores " taken at random ; but the percent- 

 age named can scarcely be said to represent accurately the pro- 

 portion 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 arenace- 

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

 usually adhere by points to the interior of the shell ; so that the 

 siliceous casts are not only far more commonly met with clear 

 of the test than the calcareous, but they are also individually 

 heavier. 



There is considerable difference in the appearance under the 

 microscope of the chambers under different conditions of infil- 

 tration. Sometimes a sphere may be found partially filled with 

 silica and partially with carbonate of lime. A section of such a 

 one is shown at PL XI., fig. 5. The lobe of silica which occu- 

 pies part of the right side of the chamber (a) is amorphous, and 

 has a yellowish tint by transmitted light, whilst the remainder 

 is calcareous and crystalline. Under these conditions it is use- 

 less to attempt a comparison between the chemical composition 



