Notices of Memoirs — ' China Clay ' a Mineral. 565 



subsoil whicli they have already purchased, and on which their 

 railway has been built. In order to determine which of these con- 

 tentions is right it is necessary to inquire into the composition and 

 origin of the china clay. As I have already said, the china clay rock 

 occurs in the granite formation only. Granite is an igneous rock the 

 mineral constitution of which differs in various places, but which may 

 be said to be practically of general uniform crystalline constitution 

 composed of felspar, quartz, and mica. Granite when exposed to 

 atmospheric or other agencies becomes decomposed to an extent 

 varying with its mineral constitution, and the first element which is 

 the subject of decomposition is the felspar. The decomposed felspar 

 results in the formation of a clayey material, and china clay rock is 

 granite in which the felspar has been wholly decomposed and replaced 

 by this clay. Different opinions are held by geologists and others as 

 to the agency by which this complete decomposition has been brought 

 about. Some contend that the decomposing agent is the carbonic 

 acid in the rain-water. Others, again, reject the rain-water, or sub- 

 aerial theory, and insist that all the known facts combine to prov6 

 that the agency by which such complete decomposition has been 

 brought about had its origin in subterranean depths, and that the 

 agent penetrated to the decomposed mass by means of cracks and 

 fissures, many of which ai"e now filled up with minerals which 

 admittedly came from lower depths. A third class of scientists, 

 represented by some of the most eminent who have given evidence 

 in this case, incline to the view that both causes — subaerial and 

 subterranean, or pneumatolytic as the latter has been called — may 

 have contributed to the result. Between these conflicting opinions 

 it is fortunately not necessary for me to decide. It is sufiicient for 

 my purpose to find as a fact that there are in the granite formation 

 in the part of the country with which I have to deal in this case 

 nests or pockets of varying superficial areas, and in most instances of 

 unknown depths, wherein is to be found a granite in which the 

 felspars have been wholly decomposed and replaced with the clayey 

 material I have already mentioned. It is further established by the 

 evidence that these nests or pockets are sporadic, and that their 

 existence adds materially to the value of the land, and that their 

 presence or absence is not to be accounted for by any apparent 

 differences in the overlying granite or other materials. As a general 

 rule it may, I think, be said that they occur under an overburden of 

 less decomposed granite, but, as I have already stated, they have been 

 found under hard undecomposed granite, and under a wholly alien 

 overburden such as the killas. Even when it occurs under an 

 overburden of decomposing granite china clay rock has, I think, 

 characteristics apart from colour which differentiate it sufficiently 

 from the overburden to enable those acquainted with the local 

 formation to fix approximately the line of demarcation between the 

 overburden and the china clay rock. The decomposed felspar — the 

 clayey substance which has replaced the felspar — constitutes, say the 

 defendants, the china clay. All we do, they add, is to extract this 

 clayey substance by a sifting or washing process, whereby we 

 disengage it from the other material with which it is found in 



