202 



MISS M. C. STOPES ON 



[May 1909, 



Though ' coal-balls ' and ' roof -nodules ' may vary considerably 

 in detail, these two analyses are representative of their average 

 structure, and show that they consist almost entirely of carbonates. 

 The ' coal-balls ' are quite free from both silica and clay, while in the 

 ' roof-nodules ' the percentage of these substances is very small. In 

 other respects, the Japanese and the Carboniferous nodules agree 

 in a general way in their chemical composition. The calcium and 

 magnesium carbonates, together with aluminium silicate, form 

 92-665 and 89-924 per cent, in the Japanese nodules, and 92-262 

 and 92*747 per cent, in the Carboniferous nodules. 



Tabulae Summary or the Principal Features of the Three Types 

 or Nodules (see p. 203). 



No animal shells in the 

 matrix. 



Matrix entirely free from 

 granules, with few 

 crystals of calcium car- 

 bonate and no quartz. 



Numerous closely-packed 

 plant - fragments of 

 many kinds. 



Chemical composition : 

 about 92 per cent, car- 

 bonates and per cent, 

 silicates. 



Japanese nodules. 



Many marine shells in 

 the matrix, largely 



Matrix extremely granu- 

 lar in most cases. 

 Numerous calcite - 

 crystals and many 

 quartz-grains. 



Some fragments singly in 

 nodules, others many 

 together in a nodule, 

 but always with much 

 matrix surrounding 

 each. 



Chemical composition : 

 about 60 per cent, car- 

 bonates and 30 per cent, 

 silicates. 



Rather irregularly - Some more or less true 



rounded masses, some J oblate spheroids in 



approaching oblate shape, others more ir- 



spheroids. regularly rounded. 



From a few millimetres From 2 inches to 6 feet 



to 2 feet in diameter, ! in diameter, 

 irregular masses much | 

 larger. 



All concretions enclosing 

 plant-fragments. 



Formed in the sur- 

 rounding layers (coal) 

 saturated with sea- 

 water. 



Roof-nodules.' 



Many concretions en- 

 closing plant - frag - 

 ments, though similar 

 ones contain only 

 shells. 



Formed in the sur- 

 rounding layers (shale) 

 under sea-water. 



Many marine shells in 

 the matrix, largely 

 goniatites. 



Matrix opaque, but very 

 fine-grained. Very few 

 crystals and almost no 

 quartz-grains. 



Fragments almost with- 

 out exception singly in 

 nodules, consequently 

 surrounded by much 

 matrix. 



Chemical composition : 

 about 86 per cent, car- 

 bonates and 6 percent, 

 silicates. 



Approximately oblate 

 spheroids. 



From 2 inches to about 

 2 feet in diameter. 



Many concretions en- 

 closing plant - frag - 

 ments, though similar 

 ones contain only 

 shells. 



Formed in the sur- 

 rounding layers (shale) 

 under sea-water. 



