GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 471 



and we obtain 800 feet as the greatest possible depth below 

 the surface of the surrounding country, and 1,100 feet as 

 the corresponding covering of shale. The minimum may- 

 be found by assuming only 50 feet denudation at Penrith, 

 and 200 feet of previous warping, giving 350 and 650 feet 

 below the general surface level of the country, and for the 

 thickness of the covering respectively. On the whole 

 perhaps some such figures as 500 and 800 are the most 

 probable, for it is extremely probable that warping of more 

 than 100 feet had taken place prior to the intrusion, whilst 

 the existence of a large flood plain of the Nepean Hawkes- 

 bury River at Penrith shows that denudation there has 

 been slow. 



II. Petrography of the Main Mass and some DirTeren= 

 tiation Products. 



By H. I. Jensen and H. S. Jevons. 

 8. General Petrology of the Mass. 

 A brief account of the constitution of the main rock of 

 the Prospect mass has already been given. This part of 

 the paper will be devoted to a detailed description of several 

 typical specimens, to an account of the characters of the 

 minerals composing the rock, and to a description of the 

 variations in composition of the rock in different parts of 

 the mass. The more important of these variations are 

 found to be a greater richness of olivine in the quickly 

 cooled envelope, with a corresponding difference in chemical 

 composition, and an increased proportion of ilmenite in the 

 lower part of the main mass, so far as exposed, and also to 

 some extent in the neighbourhood of the large aplitic veins. 

 The evidence of assimilation of the country rock, and the 

 decomposition products, including the origin of the analcite 

 will also be discussed. 



The main rock is phanerocrystalline and medium-grained 

 the average grainsize being between 1 and 5 mm. In the 



