294 iV. B. Jiinner: 



fied spherulitic tachylyte, typically coarse in structure." The 

 ■latter phrase of which was interpreted by Miss Raisin^ to prob- 

 ably mean that spherulites were visible niacroscopically. The 

 Walhalla rock therefore differs from the typical variolite in the 

 absence of macroscopic ally visible spherulites or varioles. The 

 hornblende is entirely actinolite, and it usually occurs as long 

 niicrolites frequently crossing one another. Examined under the 

 high power, many of these niicrolites are seen to be skeleton crys- 

 tals, and they very often enclose a tubular core of ground ni/ass 

 material showing very low polarisation colours. The edges of the 

 microlitea are often greatly serrated, and they usually fork at the 

 ends, and pass gradually into the ground mass of the rock. Cross 

 sections of these laths of actinolite occur in the form of small 

 jjarallelograms, with central inclusions, corresiionding to the 

 tubular inclusions present in the niicrolites. The actinolite laths 

 show a characteristic cross fracture at right angles to their length, 

 and more rarely a cleavage parallel to their elongation. Twinning 

 according to the usual law is not infrequently j) resent. Both 

 these niicrolites and the spherulites appear to be essentially con- 

 temporaneous in origin. At times the laths intersect the 

 spherulites, but often the reverse is true, and the spherulites cut 

 across, or project into the actinolite laths. Chlorite replaces much 

 of the actinolite. The felspar laths gi-ve maximum extinction 

 angles of 25^. They exhibit undulose extinction, and are serici- 

 tised in places. 



Section W8, of pebble from same locality as the preceding speci- 

 men. This is an intermediate type between the normal diabase 

 and the spherulitic rock. Microscopically, it consists almost en- 

 tirely of hornblende and felspar laths, with secondary chlorite. 

 The long niicrolites of actinolite show the characteristic serrated 

 edges, and the central tul)ular inclusion of the ground mass. One 

 or two grains of the hornblende retain the rectangular ilcavage 

 of augite, clearly proving the secondary origin of the former 

 mineral. The felspar laths are usually only simply twinned, and 

 they are occasionally zoned. Extinction angles are generally low. 

 but angles as great as 40^ were measured on isolated sections. 

 The felspar in places includes hornblende, and in others pene- 

 trates the femic mineral, showing that both minerals are essen- 

 tially contemporaneous in origin. The chlorite is markedly 

 pleochroic, in shades of green and yellow. One section gave an 



1 "The Vaiiolitic Rocks of Mont Genevre." Q.J.G S.. Loiirtoii. I8!K), p. 3:^0. 



2 " Variolite of the Lleyn and associated Volcanic Rocks." (|».J.(;.S., I-oiidoii, 1893, y>. ir>.'i. 



