23.2 STREAM TIN DEPOSITS OF PEEAK, 



LECTURE II. 



2 1st April, 1884. 



Our enquiry in this lecture will be as to the way in which we 

 can account for the rich deposits of tin ore in connexion with 

 granite, rocks. 



You will remember how, in the first lecture, we began with the 

 study of two pieces of stone, one of which was water-worn and the 

 other a rough fragment of granite. The water-worn stone fur- 

 nished us with a clue to the erosions of mountains and the forma- 

 tion of drift. We shall now turn to the rough stone to sift the 

 question of its constituent parts, and we will begin our enquiry by 

 asking — "What is granite ? 



Broadly defined, it is a compound rock consisting of quartz, 

 felspar and mica. Quartz is a very hard glassy mineral consist- 

 ing of the oxide of the clement silicon. Felspar is a trine less 

 hard and more complex. It consists of , say roughly, 60 or 70 per 

 cent, of quartz, a large percentage of alumina, and the rest made 

 up of soda or potash, and a very little iron, lime and magnesia. Mica 

 is a shiny glistening mineral, generally coloured yellow, blackish 

 or transparent. It splits into thin flakes, and looks golden or silvery 

 in small specks. Mica is a compound mineral and contains, besides 

 other minerals, notably lithia, silica, alumina and an alkali usual- 

 ly potash and magnesia, the silica being in smaller proportion 

 than in felspar. 



Now, observe that I am dealing with these things in the most 

 general sort of way. There are not only many different kinds of 

 granite but many different kinds of felspar. Granite also contains 

 other different minerals besides those which I have mentioned, but 

 exceptionally and in relatively small quantities. For my present 

 purpose, however, my definitions as above are sufficient. 



Observe ether differences in this stone. It is not stratified. 

 There are no lines nor marks such as it would have if it were a 

 rock slowly deposited by water. It is a mass of crystals. Now, 

 how did it get this form and how comes it that such a uniform ap- 

 I ranee is presented by granites all over the world? It is no 

 matt< r where you arc-— in Aberdeen, in Egypt, in Malacca, or Perak 

 — granite is granite everywhere, and every one who has eyes can 

 recognize it. 



Various theories have been proposed to account for this. I 

 cannot describe them all, but I will take the most natural and the 

 most common idea. That is, that the stone has been melted by fire. 

 The earth's surface, soil is said, is pretty uniform in materials, and 



