Relations between the Basalt and Inclusions. 



a. Sedimentary. — The most noticeable feature of this 

 group of reactions is the effect on the basalt produced by 

 the presence of xenocrysts of quartz. The grains are fre- 

 quently surrounded by a ring of exceedingly minute prisms 

 of diopside. This phenomenon has been noted in both the 

 older and the newer volcanic rock of this quarry. 



b. Igneous. — It has been suggested that the rounded 

 nature of the xenoliths was due to the corrosive effect of 

 the basalt magma. 1 It is therefore of interest to examine 

 closely the actual boundary between the basalt to seek for 

 direct evidence of such an absorption. This seems to be 

 indicated by the following facts. Angles of plagioclase do 

 not often remain sharp in the basalt, on the other hand 

 projections of augite into the plagioclase are frequent, and 

 inclusions of xenocrysts of augite may be met with further 

 from the boundary than xenocrysts of plagioclase, and are 

 more common. (See Plate 31, flg. 5.) It seems reasonable 

 to conclude, therefore that there was a strong absorption 

 of plagioclase by the basalt magma along the line of con- 

 tact, augite being absorbed less freely. When pyroxenites 

 have been shattered on their margins the basalt penetrates 

 into the cracks in long narrow veins. In such veins tiny 

 flakes of biotite sometimes form in the basalt. There is 

 also frequently a considerable segregation of magnetite or 

 ilmenite in the basalt around an inclusion. 



Hornsby Rocks. 

 The Breccia is green or greenish in colour composed of 

 a very fine grained ground mass, in which are imbedded 

 fragments of basalt or plutonic rocks, quartz grains, and 

 pebbles, calcite, bitumen, et cetera. Under the microscope 

 the breccia appears to be largely composed of quartz grains, 

 either clear and large, probably derived from a sandstone, 



1 Prof. David, Watt, and Smeeth, Proc, Roy. Soc. N.S.W., 1893, p. 403- 



