80 Geological Society of London. 
There is perhaps no coal-field in the whole country of equal size 
im which the strata have been so much dislocated and shattered. 
Mr. Prestwich gives a detailed description both of the principal and 
minor faults, their direction, extent, inclination, breadth, and fall, 
and the difference of level produced by them in their opposite sides, 
which is sometimes slight, but sometimes amounts to six hundr 
seven hundred feet. In some instances the change of level is by 
steps or hitches, which, it is truly said, may be owing either to un- 
equal resistance, or to a series of small dislocations. The walls of 
the fissures in the disjointed strata are sometimes several yards apart, 
the interval being filled with the debris of the strata. In other pla- 
ces they are in contact. In this last case it is particularly remarked 
that the surface of the ends of the fractured beds of coal and shale 
is shining and striated. You are aware that this appearance has 
usually been attributed, and I believe rightly, to the rubbing of the 
walls of the rent one against the other, the lines of the polisbed and 
striated surfaces indicating the direction of the motion, but | have 
lately seen it objected to this theory, that the stria are not always par- 
allel, but often curved and irregular, and that the earthy contents of 
veins and faults often present the same glittering and striated faces, or 
slickensides as they have been called. I am familiar with the fact, 
and have always inferred that the movements were irregular and com- 
plicated, occasionally changing their direction, and that even when 
uniform, they may have acted unequally on materials varying in hard- 
ness and pliability. It is much to be desired that scientific travelers 
who visit countries shaken by earthquakes would observe with minute 
care all the phenomena attending the fissuring of rocks and buildings. 
1 have been informed by an eye-witness of one of the late minor 
earthquakes in Chili, that the walls of his house were rent vertically, 
and made to vibrate for several minutes during each shock, after which 
they remained uninjured and without any opening, although the line 
of the crack was still visible. On the floor, at the bottom of each 
rent, was a small heap of fine brickdust, evidently produced by tritu- 
ration. In such instances it would be desirable to obtain fragments 
of the rent building, and compare them with the walls of natural fis- 
In bis examination of the fossils of the coal-measures, Mr. Prest- 
wich has shown that beds containing marine remains alternate with 
others in which fresh-water shells and land plants occur, appearances 
which he attributes to the flowing of a river, ss 6 to occasional 
