472 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



heart of the mass the grainsize is in many places as much 

 as 5 mm.; but the pallio-essexite is aphanitic, though 

 still noncrystalline, and wears the aspect of an ordinary 

 fine-grained basalt. The colour of the main mass of rock 

 varies somewhat from point to point ; for instance, above 

 the uppermost of the large aplitic veins (Plate XXXV) in the 

 Reservoir Quarry the rock is much darker in colour than 

 below. This difference in colour is probably due to degree 

 of kind of decomposition. In places the compact pallio- 

 essexite has been denuded away, allowing water to filter 

 into the more coarsely grained rock underlying it. This 

 coarser rock is much decomposed because it is permeable 

 to water through the minute contraction crevices separating 

 the crystals, and the water obtains ready access along the 

 many joints which divide it. On the other hand, the rock 

 underlying the aplitic veins is much less jointed, and is 

 accordingly less decomposed. The pallio-essexite near the 

 junction with the shale is very fresh, not having undergone 

 much jointing, and being also protected by its own com- 

 pactness, and by the band of chert into which it lias altered 

 the overlying shales. 



The specimens of which systematic descriptions will here 

 be given were chosen with great care from such localities 

 that their examination might be instructive as regards 

 changes in composition in different parts or the mass, and 

 yet that they might be taken from the freshest rock avail- 

 able. It has seemed convenient to divide the specimens 

 into three series according as they were taken : — I, from 

 the Reservoir Quarry, II, from the Emu Quarry, III, from 

 various other points on the mass. In each of the first two 

 series certain typical specimens will be taken in order from 

 the edge inwards according to distance from the nearest 

 junction, and will receive a full description, and other 

 specimens will be compared with them. The proportions 



