Ch. XV.] OF THE PRIMARY ROCKS. 355 



viously rendered flexible by some chemical agent possessed 

 of powers which we have not hitherto discovered in water." * 

 We shall hereafter revert to this curious subject : but it may 

 be now observed that this interference of one crystal with 

 another, in a compound mass, is not uncommon, though 

 seldom so distinctly displayed as in the Portsoy granite ; in- 

 deed, any large grained variety of granite, if minutely ex- 

 amined, will be found to afford some examples of this 

 phenomenon. 



The general opinion concerning the nature of veins is, 

 that they are of three kinds : 1st. Those which are referred 

 to an igneous origin, such as injected and sublimated veins. 

 2d. True veins, or such as were originally open fissures, but 

 subsequently filled by chemical or mechanical depositions 

 from water. 3d. Contemporaneous veins, or such as seem 

 to be coeval with the aggregation of the component par- 

 ticles of the rocks in which they occur. The last mentioned 

 are now often called veins of segregation : a term introduced 

 by Professor Sedgwick, at the suggestion of Mr. Whewell, to 

 express that they have been formed by a separation of parts 

 during the gradual passage of the mineral masses into a solid 

 state. 



Now if any veins of this character occur in Cornwall, we 

 may commence with them as a fixed point with whicn our 

 subsequent observations may be compared. " In all the 

 crystalline granitoid rocks of Cornwall," says Professor Sedg- 

 wick, " there are many masses and veins of segregation. Such 

 are the great contemporaneous masses and veins of shorl- 

 rock ; and some of these are metalliferous. The decomposing 

 granite of St. Austle Moor is traversed, and sometimes en- 

 tirely superseded, by innumerable veins of this description. 

 Upon these lines of shorl-rcck, there is often aggregated a 

 certain quantity of oxide of tin, which sometimes diffuses 

 itself laterally into the substance of the contiguous granite. 



* Geol. Trans., vol. ii. p. 432. 

 A A 2 



