49. _Overlap._--When the upper beds of a conformable group of strata spread over a wider area than the lower members of the same series, they are said to _overlap_. The accompanying diagram shews this appearance. An overlap proves that a gradual submergence of the land was going on at the time the strata were being accumulated. As the land disappeared below the water, the sediment gradually spread over a wider area, the more recently deposited sediment being laid down in places which existed as dry land at the time when the earliest accumulations were formed. Thus, in the accompanying illustration (fig. 16), the stratum marked 1, resting unconformably upon older strata, is overlapped by 2, as that is by 3, and so on--all the beds in succession coming to repose upon the older strata at higher and higher levels, as the old land subsided. [Illustration: Fig. 16.--Overlap.] [Illustration: Fig. 17.--Fault.] 50. _Dislocations or Faults._--When strata, once continuous, have been broken across, and displaced or shifted along the line of breakage, they are said to be _faulted_, the fissure along which the displacement occurs being termed a _fault_ or _dislocation_. The simplest form of a fault is that shewn in the following diagram, where strata of sandstone and shale, with a coal-seam, S, have been shifted along the line _f_. The direction in which the _fault_ is inclined[D] is its _hade_, and the degree of _vertical displacement_ of the beds is the _amount_ of the dislocation. Generally, the beds seem to be pulled _down_ in the direction of the _downthrow_, and _drawn up_ on the opposite side of the fault, as shewn in the diagram. Sometimes the rocks on each side of a fault are smoothed and polished, and covered with long scratches, as if the two sides of the fissure had been rubbed together. This is the appearance called _slickensides_. Slickensides, however, may occur on the walls of a fissure which is not a displacement, but a mere joint or crack. A dislocation is spoken of as a downthrow or an upcast, according to the direction in which it is approached. Thus, a miner working along the coal-seam S, from _a_ to _b_, would describe the fault, _f_, as an _upcast_, since he would have to mine to a _higher_ level to catch his coal again. But, had he approached the fault from _c_ to _d_, he would then have termed it a _downthrow_, because he would see from the hade of the fault that his coal-seam must be sought for at a _lower_ level. Faults are of all sizes, from a foot or two up to vertical displacements of thousands of feet. Powerful dislocations can often be followed for many miles across a country, running in a more or less linear direction. Thus, one large fault has been traced across the breadth of Scotland, from near St Abb's Head, in the east, to the coast of Wigtown, in the west. Every large throw is accompanied by a number of smaller ones--some of which run parallel to the main fault, while many others