12 Transactions of the Society. 



up of granules and globules of oily matter, minute granules of 

 silicious and other inorganic substances, fragments of vegetable 

 tissue, starch-globules, all connected together by a viscid cementing 

 substance of a yellowish colour which was probably the fsecal 

 matter already referred to. 



Such complex masses no doubt slowly undergo disintegration. 

 By mere attrition the organic matter on the surface would be 

 gradually removed and would form at length a very fine mud 

 which would slowly settle, while probably a smaller portion would 

 be subjected to chemical change and be ultimately dissolved. 

 Fatty matter and crystals of fatty acids are seen in plate I. 

 figs. 1 c^, 3 ; plate IL fig. 6 (? ; plate III. figs. 10, 13 d. 



Particles of Soot and Coal. 



There is no difficulty in discovering minute pieces of coal in 

 the mud, and in some instances I have found indications of 

 vegetable structure in the sections and delicate fragments of coal 

 which have accidentally resulted from the action of the water and 

 the rubbing together of particles as they were driven backwards and 

 forwards by the current. 



Much of the soft black matter present in the mud is no doubt 

 soot. Even particles of silica are sometimes found soot-stained. 

 Perhaps such black sand is derived from the smoke-impregnated 

 granite debris of the macadamized roads. Black particles of coal 

 and other forms of carbon are represented in plate I. fig. 1 d* ; 

 plate IV. figs. 10 0, 16 o, 17 o, 18. 



Diatoms. 



More than 100 dijBferent species of diatoms are found in the 

 Thames, some being peculiar to fresh water, some to salt water, 

 while the natural habitat of some specimens seems to be water 

 which is always brackish. 



The silicious skeletons of the valves of diatoms that have died, 

 and multitudes of fragments of valves in every degree of disintegra- 

 tion are found in Thames mud. After the removal of the particles 

 of sand a considerable portion of the inorganic matter of the mud 

 that remains probably consists of the debris of the valves or shells 

 of these organisms. 



As long ago as 1853 Mr. F. C. S. Koper published some 

 interesting observations on the ' Diatomacese of the Thames ' * 

 and gave a list of 104 species from the mud of the Isle of Dogs 

 alone. Of these 30 are decidedly marine, 29 belong to brackish 



* Trans. Micr. Soc. Loud., ii. (1854) p. 67. 



