Leonard Hawkes—Tridymite in Icelandic Rocks. 205 
Visible 
Altitude. depth. 
I. Primitive gneiss of Dora—Maira massif, glandular 
to granitoid 
II. Miea-schists, minute, tabular, and graphitic | 500-2,000 1,500 
eneiss with quartzite, crystalline limestone, 
steatite, graphite, and dioritic rocks 
III. Cale-schists with crystalline limestone, pes 
and amphiboliec schists . z . 1,300-2,000 700 
IY. Pietre verdi to summit of Monte Viso; : " serpentine 
and serpentinous schist ; epidotic amphibolites ; 
glaucophanic prasinites ; actinolitic, chloritic, - 2,000-3,800 1,800 
and taleose schists; euphodites, gneissiform, 
porphyritic, and schistose 
3,300 
The total visible thickness between the extreme points—exclusive 
of the fall of level in the depression between the gneiss massif and 
the calc-schist horizon on the western flank of the former—is thus 
3,300 metres. 
The fact that the mica-schist horizon flanks the gneiss massif on 
both sides, but on the eastern side along the base, viz. at a lower 
level, led Gastaldi to regard this reversal of the normal sequence as 
evidence of a zonal subsidence (sprofondamento), the more so as both 
formations dip below the valley floor and reappear about 6 kilometres 
east in the isolated outcrop of Rocca Cavour (460m.). Zaccagna, on 
the other hand, explained the phenomenon as an anticlinal retroflex 
fold of the gneiss massif from west to east. As a zonal fracture 
or subsidence, it bears close analogy to similar zonal phenomena in 
Northern Piémont, to which I shall refer in the sequel. 
(To be continued in our next Number.) 
IV.—On Trivymire and Quarrz arrer Trrpymire in IceLanpic 
Rooks. 
By LEONARD HAWKES, B.Sc. 
(PLATE IX.) 
LACROIX in his researches on volcanic rocks and their inclusions 
. has shown that tridymite occurs in two distinct forms, each 
produced under special conditions. ‘‘ Il est important de constater 
dans un méme échantillon l’existence de tridymite produite par deux 
modes de genése distincts ; qui lui ont imprimé deux formes différentes. 
L’une Welles est caracterisée par des cristaux épais, a macles binaires, 
elle a été formée par fusion; l’autre se présentant en lamelles ex- 
trémement minces, empilées a été produite par action pneumatolytique. 
Cette variété de tridymite constitue le dernier des minéraux dr rusiques 
formés, et elle résulte trés probablement de l’attaque a haute 
température dun résidue de matiére vitreuse trés siliceuse dont des 
restes peuvent étre parfois encore constatés directement’ (1, p. 387). 
Both forms occur in Icelandic rocks. 
In his description of the liparite of Hlidarfjall, Backstrom notes 
‘‘sehr schonen Tridymite als letzte Bildung” (2, p. 661). Through 
the kindness of Professor Backstrom I was enabled to examine his 
