496 General McMahon — Farther Remarks on Granite. 



porpliyritic crystals of felspar, wLicli in some places on Dartmoor 

 attain a length of from three to four inches. I also referred to zonal 

 crystals as evidence of slow and gradual crystallisation, and I specially 

 referred to zonal felspars which show a progressive change of species, 

 from a more basic one at the heart of a crystal to a more acid one 

 at its periphery. I explained this by referring to a law of general, 

 though not of universal application, that the more basic minerals 

 crystallised first, and that in a granite there is a progressive silicifi- 

 cation of the magma as the basic minerals crystallise out, until at 

 length nothing but free quartz remains. It is obvious that the 

 gradual silicification of a plutonic rock must have been a process that 

 extended over a considerable period of time, and in the case of a 

 cooling granite there must have been a fall of temperature during 

 this period. The crystallisation of a granitic mineral may there- 

 fore have begun at one temperature and have concluded at a much 

 lower temperature. This is the more probable, as recent experiments 

 on the melting-point of minerals appears to show that a period of 

 plasticity precedes the development of rigidity in some minerals. 



In the case of the beryl of the Satlej granite I certainly con- 

 sidered that its crystallisation had been a slow and gradual process. 

 I showed that it was the first mineral to crystallise, and did so 

 before the mica, the felspar, and the quartz had begun to form 

 upon it. It seemed clear from the idiomorphic character of the 

 beryl that the magma when it began to crystallise out was in 

 a fluid condition, and I thought, as stated in my address, that 

 a study of " thin slices of it under the microscope ought to give 

 us a clue to the condition of the magma at the time the beryl was 

 formed." The beryl contained, as I found by its microscopic study, 

 not only inclusions of vi^ater, but inclusions of a gas. The evidence 

 therefore seems to show that if the molecules of beryl entangled 

 and brought down with them a gas or gases, there was no reason 

 vphy they should not have brought down water in the gaseous state. 

 That a molten magma at the heat supposed, rendered fluid by 

 aqueo-igneous fusion, must have been in a perfectly mobile condition, 

 permitting of the free motion of its constituents and an intermixture 

 of its chemical elements, goes without saying. 



The evidence submitted to the hearers of my address proves, 

 I contend, that when the atoms of the chemical substances that 

 constitute beryl came together to form molecules, and moved 

 towards each other in the act of crystallisation, they entangled and 

 brought down gases with them. There is nothing inherently 

 improbable in this conclusion. 



Those who have tried their hand at chemical analysis will under- 

 stand how difficult it is to prevent a precipitate thrown down by 

 chemical action from bringing down with it other substances which 

 the chemist is anxious to retain in solution, repeated re-solution and 

 re-precipitation being necessary in some cases where exact results 

 are required. The power possessed by some metals at a high 

 temperature of 'occluding' or absorbing hydrogen gas affords 

 another highly suggestive illustration of molecular entanglement. 



