represented in Fig. 303 shows that the fissure had its two walls : 
coated first with the layers b 6. ‘Thereafter the still open or 
subsequently widened cleft received a second layer (ee) on each face, 
and so on progressively until the whole was filled up or until only 
cavernous spaces (druses) lined with crystals were left. In such 
cases no evidence exists of any terrestrial movement during the 
process of successive deposition. ‘The fissure may have been origin- 
ally as wide as the present vein or may have been widened during 
the accumulation of mineral matter so gradually and gently as not 
to disturb the gathering layers. But in many instances, as above 
stated, proofs remain, of a series of disturbances whereby the 
formation of the vein was accelerated or interrupted. Thus at the 
Wheal Julia lode, Cornwall, the central zone (e in Fig. 304) is 

OPENINGS OF THE SAME FISSURE (B.). 
a f f, Copper-pyrites and Blende; 6b d e, h, 7, Quartz in crystals pointing inwards ; 
c, clay; g, empty space. 
formed of quartz-crystals pointing as usual from the sides towards 
the centre of the vein, but it is only one of five similar zones, each of 
which marks an opening of the fissure and the subsequent closing of 
it by a deposit of mineral matter along the walls."| The occurrence 
of different layers on the two walls of a vein may sometimes indicate 
successive openings of the fissure. In Fig. 305 the fissure at one time 
3 
| peepee 
| I OSES 
I oo EE 
= AY Bree NIRS 
| AER Biloce 
aie Pegragem hs Sin, Oe at, 
a Cc d 

Bia. 305.—Sxrorion or part or A Lopx, Goporpuin Bripan, Cornwaun (B.). 
a, Quartz coating cheek of vein; b, Quartz Crystals pointing inward ; 0, Agatiform 
Silica ; d, thick layer of Copper-pyrites. 
no doubt extended no farther than between 1 and 2. Whether 
the band of copper pyrites had already filled up the fissure previous 
' De la Beche, Geol, Obs. p, 698. 
Mey 
f 
594 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox 1V. 
