232 TRANSITION LIMESTONE. GREY WACKA. 
are generally disposed in horizontal or flat strata. Some 
of the latter contain the fossil remains of marine ani- 
mals and shells, approaching in character and appear- 
ance to the kinds which are now found in the ocean ; 
and others contain shells precisely similar to those now 
known to exist. These rocks usually occur at the foot 
of primitive mountains, or in deep valleys. 
1. TRANSITION ROCKS, 
265. TRANSITION LIMESTONE is distinguished by 
containing marine petrifactions of corah, and other zoophytes 
which are supposed no longer to exist. It often contains veins 
of calcareous spar, and exhibits a variety of colours, which give 
to it a marbled appearance. 
This species of limestone occurs in immense beds, 
and forms a great portion of the mountainous parts of 
Derbyshire and Scotland ; but it does not rise so high, 
on the sides of mountains, as primitive rocks (250). 
It often contains veins of valuable metallic ores. 
When cut and polished, many of the varieties of tran- 
sition limestone are beautiful marbles ; some of them 
have been already described. 
266. GREY WACKA is a transition rock, composed of 
pieces of quartz (76), flinty slate, felspar (110), and clay 
slate (120), cemented together ly a basis of clay slate. 
It has various appearances, the pieces being sometimes as 
large as a hen's egg, and sometimes so small that they can 
scarcely be perceived by the naked eye. 
When the rocks of grey wacka are not covered by 
those of any other formation, they form round-backed 
hills, usually insulated towards the tops, and intersected 
by deep valleys. They are widely distributed : and are 
often extremely rich in ores, both in beds and veins. 
Almost all the mines of copper, lead, and zinc, in the 
Hartz, are in grey wacka ; and, in Transylvania, this 
species of rock is traversed by numerous small veins of 
gold. 
