590 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
variously classified; but for the purposes of the geological student 
it is most convenient to consider them from the point of view of 
geological structure and history. ‘Thus arranged, they naturally 
eroup themselves into three great series: Ist, those contempora- 
neously deposited among stratified formations; 2nd, those contem- — 
poraneously formed with the other ingredients of crystalline (massive 
and schistose) rocks; 3rd, those subsequently introduced by infiltra- 
tion or otherwise into fissures, caverns, or other spaces of any kind of 
rock. 
1. Contemporaneous ores of stratified rocks have been 
deposited in water together with the sandstones, limestones, or other 
strata among which they lie. They belong to the stratified type of 
geological structure described in Part I. (p. 474). They occur in 
beds varying from mere films up to masses of great thickness. In 
some cases they retain the same average thickness for long distances, 
in others they swell out or die away rapidly, or occur in scattered con- 
cretions. Among the more frequent ores of this group are limonite 
and siderite. Abundant examples are supplied by the bog-iron de- 
posits now forming, and by the bands of brown-iron ore, red-iron ore, 
and clay-ironstone associated with Carboniferous and other formations. 
Occasionally the ore has been finely disseminated through the strata — 
at the time of their deposit, asin the cupriferous slates of the German 
Zechstein. Organic remains are commonly associated with ores of 
this type (ante, p. 174). 
. Contemporaneous ores of crystalline rocks are ex- 
emplified by the beds of iron-ore, pyrites, &c., that so frequently occur 
intercalcated among the crystalline schists (ante, pp. 118, 569). 
They lie as massive sheets or thin partings, and usually present a 
conspicuously lenticular character. That they were formed con- 
temporaneously with the layers of quartz, mica, felspar, hornblende, 
or other minerals among which they lie, may usually be inferred with 
considerable certainty, though cases not infrequently arise where it 
is difficult or impossible to draw any line between this type and that 
of true subsequently-formed veins. Besides these lenticular ores of 
the crystalline schists, the massive rocks also contain contempora- 
neously crystallized ores. ‘The diffused magnetite and titaniferous 
iron of the basalts, diabases, &c., are familiar illustrations. Large 
included masses of these and other ores are sometimes available for 
mining (ante, pp. 64, 145, 147). 
3. Subsequently introduced ores are distinguished by the 
contrast between their contents and structure and those of the rocks 
through which they pass. They have been deposited, subsequent to 
the consolidation of these rocks, in cavities previously opened for 
Lead Ores,” 1861. Numerous valuable papers by the late J. W. Henwood and others 
are to be found in the Trans. Roy. Geol. Soc. Cornwall. It is understood that a syste- 
ihatic Mnglish treatise on the subject may be expected from Mr, J: A. Phillips and 
Mr, Ji, Bauerman. 
