SILL STRUCTURE AND FOSSILS IN ERUPTIVE ROCKS. 287 
matrix, while indistinct outlines of large pebbles of slate and 
sandstone clearly point to the fact that it is an altered sedimen- 
tary rock, the rearrangement of the particles with the production 
of the crystals of felspar and quartz being due partly to chemical 
action, and partly to heat and pressure caused by the shrinkage 
of the earth’s crust.” 
Mr. C. 8. Wilkinson, the late Government Geologist of New 
South Wales, was inclined to consider crystalline rocks, such as 
those of Hill End, which contained distinct traces of pebbles as 
being highly metamorphosed conglomerates, and the comparative 
absence of metamorphism from the fine grained strata between 
these pebbly crystalline rocks he considered was due to selective 
metamorphism. These views he explained to me in the field when 
we examined the tin-bearing quartz-porphyries of New England, 
at Emmaville in 1883. Quartz-porphyries were observed by us 
at Rose Valley and elsewhere near Emmaville to contain water- 
worn pebbles of other rocks scattered throughout them. The line 
of strike of the pebbles cuts somewhat obliquely across the trend 
of the dyke or sill of quartz-porphry, and, on tracing it beyond 
the limits of the sill, we found that in either direction it passed 
into a typical conglomerate, the pebbles of which were set ina 
sedimentary base instead of a base of quartz-porphyry. It did not 
escape the eye of so keen an observer as Mr. Wilkinson, that 
selective metamorphism was incapable of explaining all these 
phenomena, and he directed my attention specially to further 
investigating this point when studying the geology of the 
Vegetable Creek district. I was, however, unable to obtain a 
Satisfactory explanation until the year 1890, when a clue was 
given by the geological structure of the Junction Gold Mines 
near Mandurama, N. S. Wales, examined by me in that year. 
A note on the remarkable structure of the eruptive rocks at the 
above gold mine was contributed by me at the time to the Linnean 
Society of N.S. Wales.! 
1 Proc. Linn. Soc. N. S. Wales, (Series 2nd) Vol. v., pp. 421 - 424. 
