50 



Altered Rocks. [No, 11, new series. 



4th Period. The crystals of Quartz are perfectly vitrified when 

 in absolute union with the Trap, and by intimately blending with 

 it, contribute to form Sydian stone and other Siliceous Traps. As 

 they recede in distance from the molten matter they are neces- 

 sarily in a lower degree of fusion. The effect of the dynamical 

 action will therefore only tend to elongate their crystals, throwing 

 the longer ones into a parellelism with its direction. They are all 

 probably however in a state of onward motion, a motion that 

 diminishes in intensity according to the distances of the crystals 

 of Quartz from the motive cause. Figure 3 will explain this. 



5th Period. At this period the passage of the Trap through the 

 fissure suddenly ceases. Pressure acts upon the rock and the 

 stratified Granite not yet cool is consolidated into a Siliceous 

 Schist, or to be more plain, the elongated crystals of Quartz in the 

 rock are flattened by pressure and brought into almost immediate 

 contact with each other, while the more easily yielding Felspar is 

 squeezed out between them, a very small stratum of it remaining. 

 Any person who could have the opportunity of observing the 

 altered rocks of the Neilgherries would find they present appear- 

 ances corroborative of the foregoing history. With reference to 

 the first period there are many rocks pierced by Trap, the sides of 

 whose rents are unaltered, because its injection ceased before they 

 had time to melt. 



Referring to the 3rd Period, Geologists are well acquainted 

 with what has been termed the passage of Granite into Trap, the 

 difficulty in accounting for which I think I have surmounted. 

 Dr. MacCulloch has stated he has observed the passage of 

 Granite into Trap. I should rather say he observed an illustration 

 of my theory. 



Illustrations of cooling in the crystals having taken place at the 

 conclusion of the 4th Period when no pressure operated, may be 

 seen in the altered Syenite and Granite rocks of the Hills. Gra- 

 nite passing into a Schistose Granite, and the latter altered into 

 Siliceous Schist, may likewise frequently be seen. The modified 

 form of Trap into Trap Porphyry which occurs traversing crystalline 

 rocks in thin veins, may in a great measure depend on similar 



