ORES OF FIVE MINOR METALS 121 
its age. The traditional view assigned it to the Hercynian 
movements mainly on the ground that the cinnabar must 
have been introduced before the sandstone was altered into 
quartzite. This change was probably during the Upper 
Silurian (Caledonian) movements, as the Devonian conglomer- 
ate includes pebbles of Ordovician quartzite; and the occur- 
rence of the cinnabar along the joints, and around angular 
fragments of quartzite, and replacing quartz on lines crossing 
adjacent grains, shows that the rock was quartzite before 
the introduction of the mercury. Mercury ores, from the 
evidence of the chief fields, are formed nearer the surface 
than the rocks of the Almaden Mine could have been in 
Upper Palzozoic times. Hence if the cinnabar there were 
Paleozoic in age, it should have been redeposited in the 
Kainozoic, and its abnormal richness suggested that the 
present ore is a secondary concentration. Much of the San 
Pedro Lode yields 30 per cent. of cinnabar; the average yield 
at Almaden was 7 per cent.; whereas that from California 
was about +3 per cent., that of Idria '7 per cent., and of Mt. 
Amiata in Tuscany under 1 per cent. The Almaden lodes, 
however, show no evidence of the secondary origin of the 
bulk of the ore. 
The analogy with other mercury mines is in favour of the 
geologically modern origin of the Almaden ore;! and this 
view is adopted by the most recent Spanish authority,? which 
assigns the ores to hydrothermal injections at the end of the 
Kainozoic, and after the Alpine (i.e. Oligocene and Miocene) 
movements. The dependence of the ores on presumably 
Alpine faults is shown at the eastern end of the mine, where 
the main lode ends against a nearly vertical fault that throws 
the rocks southward; Almaden agrees with other leading 
mercury fields in the occurrence of its ores in beds greatly 
disturbed by Middle Kainozoic mountain movements; but 
the structure is comparatively simple as the beds have been 
fractured and not greatly overthrust. 
IDrRIA—At Idria, near Trieste (Fig. 37), the mercury mines 
are in an area of geological interest owing to the early date at 
which thrust planes were proved there. Lipold in 1874 (¥akrb. 
k. k. Geol. Reichsanst., xxiv, pl. 10, Fig. 1) published a section 
! Cf. Gregory, J. Chem. Soc., cxxxi, 1022, p. 769. 
* Minas de Almaden, Geol.. Congr. Internac., Madrid, 1026, pp. 67-8.