66 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Mud and Clay. 



I do not see how it would be possible to ascertain the true 

 nature of the rock which gave rise to the kaolin or other minute 

 granules met with in mud or clay, unless it were by observing 

 their connexion with other associated materials. As far as the in- 

 dividual particles are concerned, we can do little more than distin- 

 guish between more or less irregular granules, minute flakes of 

 mica, and needle-like prisms. In many cases it would be difficult or 

 impossible, even to decide to what mineral species such minute 

 granules belong ; and in using the term mica, it must be under- 

 stood that I do not mean any one particular mineral, but rather 

 any of those minerals which have the same laminar structure as 

 mica, and an axis of elasticity perpendicular to the laminae, along 

 which the double refraction is negative. Still, notwithstandiug 

 these difficulties, a good deal may be learned in one way or other. 



The most striking differences between different specimens of 

 mud and clay depend on a greater or less variation in the amount 

 of mica, calcareous granules, and sand mixed with the unidenti- 

 fiable granules ; but these differences do not necessarily enable us 

 to trace the material to its true source. Larger grains of com- 

 pound rocks, either in the clay itself or in closely associated beds, 

 may, however, furnish good evidence of the true origin of the 

 material, if proper allowance be made for the hardness of the 

 various rocks or minerals. It is very important to do this, since 

 the larger compound fragments may represent only the very hardest 

 rocks, and the great mass of the fine-grained mud may have been 

 derived from associated soft rocks which have been completely 

 disintegrated. 



It would extend this address to an unreasonable length to describe 

 in detail the structure of our various groups of stratified rocks ; 

 and before any general conclusions could be formed it would be 

 necessary to examine a very extensive series of specimens. Still 

 the method I now describe does seem to lead to some valuable 

 results, even though I have been able to apply it to only a limited 

 extent. 



In the more modern strata, down to the Devonians, the sandy 

 beds of which are mainly composed of quartz grains derived chiefly 

 from granite when coarse, and from schists when fine, the asso- 

 ciated clays and shales very commonly contain scattered grains of 

 similar sand, but are chiefly composed of minute granules of de- 

 composed felspar, flakes of mica, and the other minute particles 

 already mentioned. Compared with the deposits of the deep 

 oceans, many of our more modern fine-grained mud deposits are 

 characterized not only by the absence of minute calcareous or- 

 ganisms, but also by the comparative absence of pumice and other 

 true volcanic products. I hesitate to affirm that pumice is com- 

 pletely absent. It certainly does occur abundantly in some of the 

 Carboniferous strata of Scotland, which are associated with erupted 

 rocks ; and, judging from its remarkably wide distribution in the 



