and Igneous Rocks of Ensay. 67 



Ensay. Their great display is on the western side of the 

 Tambo River, and thence to Castle Hill, which is the termi- 

 nation in that direction of the section accompanying this 

 paper. They there occur in wide tracts, broken in places by 

 the exposure through denudation of intrusive quartz diorites. 

 The only part of the western moiety of the section which 

 now needs any special reference is that portion which is at 

 the western side of the Tambo, and where the Silurian 

 sediments are well shown. 



The Silurian rocks at this place are highly inclined at 

 angles between 70° and 90° dipping to S. 20° to 30° W. 

 This formation is continuous fco Mt. Baldhead westward, 

 and northward to the Gum Forest, which is part of the 

 Swift's Creek intrusive area. 



On descending from these hills towards the river the 

 boundary of the invasive rocks is reached, at an elevation of 

 about 300 feet above the stream, but the actual contact is 

 not visible on the line of section. 



I collected a number of samples of these sediments, and 

 examined them, with following results. Speaking generally, 

 they fall into two classes, representing the normal argilla- 

 ceous and quartzose Silurian beds; but they differ from them, 

 in so far that they have all been more or less metamorphosed. 



Of the collected samples, I selected two for special exami- 

 nation. That which represents the argillaceous sediments is 

 a minutely -spotted schistose rock, inclined to slaty cleavage, 

 of a greenish-grey colour, a slightly silky lustre, and with 

 here and there minute plates of alkali-mica, to be seen under 

 the pocket lens in the otherwise crypto-crystalline mass of 

 the foliations. 



Under the microscope, thin slices of this rock showed that 

 it is composed of two kinds of mica, together with granules 

 of quartz and some black material (opacite). The spots are 

 entirely composed of minute flakes of colourless alkali-mica, 

 with some black material. The main mass of the rock is a 

 mixture of the two micas, of which the second is a brown, 

 magnesia-iron mica, which in places predominates, just as the 

 alkali-mica does in others. The difference between the two, 

 however, is that the brown mica occurs in the mass of the 

 rock, and also radiates from the exterior of the spots. The 

 quartz is in minute rounded grains. Here and there I 

 observed colourless rod-like microliths, which must be 

 apatite, and also rather stout colourless microliths, with 

 oblique terminations. These have the form of tourmaline ; 



f2 



