196 Dr. Wyatt Wingrave—A New Variety of Ammonite. 
fusing in the neighbourhood of 1,375° C., has barely been affected ; 
quartz, fusing at 1,625° C., has only suffered fracturing. We may 
therefore conclude that the temperature of the basaltic magma during 
intrusion was between 1,170° C. and 1,375° C., excluding, as seems 
legitimate, any chemical reaction between orthoclase and the 
pitchstone glass. The ordinary basalt lava is fusible at about 
1,100° C., and this is perhaps the ordinary temperature of emission 
at a volcano, although Daly believes temperatures of 1,200° C., or 
even 1,300° C., may prevail in the great volcanoes of Hawaii at 
periods of intense activity." The fusion of minerals and even rocks 
has been observed in basaltic magmas by Lacroix and others.’ 
Independent evidence of high temperature upon intrusion in the case 
of the dyke in question is afforded by its tachylytic selvages. These 
are due to very rapid marginal cooling, and the rate of cooling 
depends directly on the difference between the temperature of the 
country rock and of the magma. 
The occurrence of xenoliths of pitchstone in a Cumbrae type of 
basalt, which itself intrudes a sill of teschenite or crinanite, affords 
useful evidence as to the relative ages of these three groups of 
intrusive rocks. A pitchstone dyke with a felsitic centre cuts the 
Dippin crinanite near Torr an Loisgte, about 13 miles north-west of 
the quarry in which the xenoliths are found.s This dyke trends in 
a north-west direction, and would therefore, if produced, appear 
in the vicinity of the quarry, beneath the crinanite sill. It is 
possible that fragments of this dyke have been brought up by the 
basaltic magma. ‘The sequence of the three types in time is clearly, 
first the crinanite sill, then the pitchstone, and finally the Cumbrae 
basalt. Similar evidence from other parts of the island goes to show 
that the Cumbrae basalts were one of the latest manifestations of 
igneous activity in Arran, if not the latest. 
IIl.—A New Variety or THE AmmonitR Ca@ztocrrés DAV 41, 
FRoM tHE Lower Lis, Dorser. 
By WYATT WINGRAVE, M.D. 
(PLATE VIII.) 
UENSTEDT ‘ described three distinct varieties of Ammonites 
Dave, with nine illustrations. They all belonged to the y zone 
of Lower Lias :— 
1. Ammonites Davai, whose tubercles are somewhat clavate and 
with unsymmetrical prorsiradiate ribs. 
2. A. Davei enodis is continental. It is a small specimen without 
tubercles, with fine symmetrical and prorsiradiate ribs. 
3. A. Davei nodosissimus is also continental. This variety shows 
nine large bullate tubercles on each whorl, with close, fine, 
and symmetrically disposed flexiradiate ribs. 
1 Tgneous Rocks and their Origin, 1914, p. 212. 
2 Tiacroix, Les Hnclaves des Roches Voleaniques, 1893, pp. 563-5. 
5 A. Scott, ‘‘Pitchstones of South Arran’’: Trans. Geol. Soc. Glasgow, 
vol. xv, pt. i, p. 22, 1914. 
4 Quenstedt, Die Ammoniten des Schwdbischenjura, 1885. 
Sate 
eee pS eee 
