GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 453 



showing the junction of the igneous rock with the shale on 

 the periphery of the mass is anywhere exposed, hence only 

 hypotheses can be advanced concerning its exact position, 

 or angle of dip, at either end of the section. At the west 

 end there is, however, some evidence, for shale was found 

 in situ at the top of the bank of the cutting. Furthermore, 

 one of us was assured by the Waterworks Engineer, Mr. 

 Jacob, that the shaft which is on the line of the section AB 

 was sunk entirely through shale. As an exposure of pallio- 

 essexite occurs something less than 100 yards to the east 

 of where the shale is found in situ, and as it is found from 

 this point to follow the slope of the hill upwards until it 

 gives place to the essexite, it seems rational to suppose that 

 here the envelope of compact rock is dipping outwards, i.e. 

 west, and at an angle a little steeper than the slope of the 

 hill. If this be so, the junction with the shales must dip 

 more and more steeply as it descends or else it would not 

 avoid the shaft which is 50 feet deep. Probably the peri- 

 phery of the mass has the vertical section shown by the 

 hypothetical broken lines at the western end of the section. 

 At the eastern end of the section, although there are a 

 few exposures of pallio-essexite on the outer slope of the 

 hill, there is not such clear evidence of the direction of 

 the junction. 



The position of the outer junction is difficult to determine 

 on almost every side of the mass ; but we have been to some 

 extent guided in mapping by a rather sudden change of 

 surface gradient from a steeper slope above to one more 

 gradual below, which probably indicates roughly the change 

 from harder to softer rock. At first considerable use was 

 made of the change from the red soil of the igneous rock to 

 the buff-coloured soil of the shales. This change is empha- 

 sised by the occurrence where the soils mix of peculiarly 

 shaped calcareous nodules varying in size from that of a 



