THE SCOPE OF ECONOMIC GEOLOGY 9

horizonta] fissures due to the shrinkage of the rock during
cooling, The floors are arranged like the rungs of a ladder,
and such lodes are called ladder-lodes (Fig. 3). The typical
®Xamples are at Wood's Point in Victoria, where they
Occur in dykes of hornblende-porphyrite in slate. As a
fule the floors of a ladder-lode are confined to the igneous
tock ; but where on solidification that rock froze firmly to
the adjacent slate the shrinkage cracks and consequently
‘he quartz-floors extend into it.
Contra-lodes (Fig. 4) are small lodes which cross a lode at
a high angle, just as great faults are crossed by secondary
Cross-course faults. If some lode material has been deposited
along a cross-fault it is a metalliferous cross-course or contra

Cu
¥

Cus

F16. 3.—A LADDER-Lopg, F16. 4—A ConTra-Lobk.
A ladder-lode in a dyke traversing A contra-lode (Pb) containing
slate. In two cases the quartz- lead, formed along a fault
floors are shown penetrating which has broken a copper
along cracks into the slate. lode {Cu).

lode ; if the cross-course be only filled with clay it is a crossfluccan.
 Great faults may extend to depths of probably
a hundred miles, and the fissure lodes doubtless extend far
below the levels which can be reached by mining ; and
some lodes, or series of associated lodes. may extend for
hundreds of miles in length.
Lodes are usually much longer than their thickness ;
but those formed along the intersection of two fractures or
in a solution channel are pipe-lodes or * ore chimneys,” as
the Achilles lode of gold ore at Tarradale, Victoria, or the
Harrington-Hickory Mine in Utah due to the replacement of
limestone along a cross fissure by lead ore (Butler, USGS.
Prof. Pap., 111, 1920, p. 517).
Lodes in folded sedimentary strata are often isolated. and