236 
PETROLOGY OF LOCAL PEBBLES. 
Oct., 1889. 
W. H. Flower: “Notes on Four Specimens of the Common Fin 
Whale,” Proc. Zool. Soc., 1869, p. 604 and plate. 
Robert Heddle : “ On a Whale of the Genus Physalus,” Proc. Zool. 
Soc., 1856, p. 187. (Plates of Common Rorqual.) 
R. Collet: “ On the External Characters of Rudolphi’s Rorqual,” Proc. 
Zool. Soc., 1886, p. 243 ; (two plates of this species; also engravings 
of Balcenophilus, Echinorhynchus, and Calanus). 
Robert Broivn : “ Notes on the History and Geographical Relations of 
Cetacea frequenting Davis Strait and Baffin’s Bay,” Proc. 
Zool. Soc., 1868, p. 533. 
Robert Gray : “ Notes on a Voyage to the Greenland Seas,” Zoologist, 
1887, pp. 49, 94, 121 (plate of B. mysticetus); 1889. pp. 1, 41, 95. 
E. Harting : “ Distinguishing Characters of British Cetacea,” Zoologist, 
1878, p. 1. 
Escliricht, Reinhart, and Lilljeborg : “ Recent Memoirs on the Cetacea,” 
Ray Soc., 1866. 
THE PETROLOGY OF OUR LOCAL PEBBLES. 
BY T. H. WALLER, B.A., B.SC. 
(Concluded from page 219.) 
On examining a thin slice, however, a well-marked micro- 
granitic structure becomes visible, not of a very minute 
character, the individual portions of quartz and felspar being 
some -001 to -002 of an inch in diameter. The porphyritic 
crystals of felspar are all orthoclase. They are very cloudy, 
and in one or two places are invaded by the tourmaline, as if 
we saw the beginning of the process what has ended in the 
last-named pebble in the formation of a tourmaline aggregate 
replacing the entire felspar crystal. The tourmaline crystals 
are distributed along lines in the felspar substance, apparently 
the traces of what Prof. Judd terms solution planes, along 
which the alteration of crystals frequently takes place. 
Where the change is complete the banded arrangement is 
still visible, in such a way as if along these planes radiating 
masses of the tourmaline had penetrated into the felspar 
until the whole was a mass of interlacing crystals. 
The tourmaline is not in most cases in the large crystal¬ 
line masses previously mentioned, but much more minutely 
and confusedly crystallised. In one case long arms of 
crystals radiate from a centre, the axes of the crystals being 
parallel to the length of the arms. 
The porphyritic quartz is crowded with relatively large 
cavities, frequently arranged in lines, which appear to be 
