May 1861. j 



Altered Rocks. 



\9 



V. — Observations upon the altered Rocks of the Neilgherries. By 

 Major H. Congreve. 



In the course of some papers recently written by me on the 

 Geology of the Neilgherry Hills, I had occasion to observe that a 

 crystalline rock of Syenite pierced by Trap was in part converted 

 into a Schistose rock. Since then I have had many opportunities 

 of observing the action of Trap on the rocks it has perforated, 

 particularly Syenite and a Granite of two ingredients, viz., Quartz 

 and Felspar. The sides of the fissure in many of the Granite 

 rocks of this composition, subsequently filled by igneous injection 

 are altered into a lamellar Granite and Siliceous Schist. 



Having carefully studied the mineralogical arrangement of the 

 crystals of the Schist, Schistose Syenite and Granite and the sub- 

 ject generally, I am induced to believe that the following is a true 

 history of the change. To make it more cle<*r I divide the opera- 

 tions wrought by dynamical and mechanical forces into several 

 periods illustrating each by a diagram. 



Period the first. In this early stage of the phenomena we sup- 

 pose a mass of Granite rock composed of Quartz and Felspar to 

 have been rent by some natural convulsion ; and the rent to offer 

 a free duct for the discharge of certain volcanic substances from 

 the interior of the earth to its surface, such molten fluids not hav- 

 ing yet brought the sides of the chasm through which they pass 

 into a liquid state. 



2nd Period. The incandescent fluid having continuously passed 

 through the fissure has at length imparted to the sides of it, so 

 great a degree of heat as to produce a state of fusion amongst its 

 minerals : the Quartz not being so highly fused as the Felspar 

 owing to its greater degree of infusibility. 



3rd Period. By this time the sides of the chasm have become 

 so much liquified by the heat of the injected substances as to lose 

 all adherence; and the dynamical force of the trap rushing 

 through the fissure acts upon the molten rock which environs it. 

 The Trap in other words blends with the molten Granite at the 

 points of junction and carries it forward with it. 



