ANNIVEESAET ADDEESS OP THE PEESIDENT. 67 



modern oceanic deposits brought back by the ' Challenger/ I am 

 quite prepared to meet with it occasionally in many of our strati- 

 fied rocks, which I haye not yet adequately examined to decide 

 this special point. 



Deposits mainly composed of inert substances like quartz, mica, 

 and kaolin can undergo little further change. The complete 

 decomposition of the felspar, however, may not occur until after 

 the material has been deposited; and its further decomposition 

 may give rise to quartz or opal; and the decomposition of the 

 hornblende and analogous minerals may also give rise to silicates 

 of iron and other products, formed in situ. The amount of such 

 changes does not appear to have been very considerable in the 

 case of most of our more recent English rocks ; but still I think 

 we must attribute to it some well-marked characters, and a more 

 or less considerable share in cementing the grains together to form 

 a hardened rock. It is more especially in the case of volcanic 

 ashes that we should expect to find this sort of action at a maxi- 

 mum, since they are often to such a large extent composed of 

 unstable minerals, ready to undergo great changes w^hen sur- 

 rounded by wet mud. 



Lamination of Shales, ^c. 



I am sure that most geologists must have been struck with the 

 great similarity between the laminar fracture of many shales and 

 the cleavage of imperfect slates. It is, in fact, a sort of cleavage 

 in the plane of stratification. I do not refer to thin beds of 

 different mineral character in the plane of deposition, but to the 

 more irregular lamination of thick-bedded shales. 



In order to throw light on this question I made a careful ex- 

 periment with some fine-grained mud from 2600 fathoms in the 

 South Pacific, collected by the 'Challenger.' Such fine-grained 

 muds have a very peculiar property, which must play an important 

 part in their origin and structure. "When suspended in water the 

 grains of sand which they contain do not separate and subside 

 quickly, and leave the fine mud suspended for a long time ; but the 

 coarser and the finest particles rapidly coalesce into compound 

 granules, which subside at a more uniform and intermediate rate, 

 soon leaving the water clear. This property readily explains why 

 such fine-grained mud so often contains grains of sand. By 

 gentle agitation the coalescence may be to some extent prevented. 

 Now, having mixed some of this Pacific mud with water, and 

 having kept it until no further subsidence occurred in several 

 weeks, I determined the amounts of water and solid matter in the 

 stiff pasty deposit, and found that the actual bulk of the solid 

 matter was only 11 per cent, of the whole, and that the spec. gray, 

 of this solid matter was 2*65. As far as I can judge, the volume 

 of the solid matter in shales containing no infiltrated matter may 

 be considered to be at least 75 per cent. ; and hence the squeezing- 

 out of the water from material like the simply deposited Pacific 



?2 



