38. _Beds, their Extent, &c._--Beds of rock are not only of very different thicknesses, but they are also of very variable extent. Some may thin gradually away, or 'die out' suddenly, in a few feet or yards, while others may extend over many square miles. Beds of limestone, for example, can often be traced for leagues in several directions; and if this be the case with certain single beds, it is still more true of groups of strata. Thus the coal-bearing strata belonging to what is called the Carboniferous period cover large areas in Wales, England, Scotland, and Ireland, not less, probably, than 6000 square miles; and strata belonging to the same great period spread over considerable tracts on the Continent, and a very extensive area in North America. It holds generally true that beds of fine-grained materials are not only of more equal thickness throughout, but have also a wider extension than coarser-grained rocks. Fine sandstones, for example, extend over a wider area, and preserve a more equable thickness throughout than conglomerates, while limestones and coals are more continuous than either. 39. When a bed is followed for any distance it is frequently found to thin away, and give place to another occupying the same plane or _horizon_. Thus a shale will be replaced by a sandstone, a sandstone by a conglomerate, and _vice versâ_. Sometimes also we may find a shale, as we trace it in some particular direction, gradually becoming charged with calcareous matter, so as by and by to pass, as it were, into limestone. Every bed must, of course, end somewhere, either by thus gradually passing into another, or by thinning out so as to allow beds which immediately overlie and underlie it to come together. Not unfrequently, however, a bed will stop abruptly, as in fig. 3. [Illustration: Fig. 3.--Sudden ending of Bed at ×.] 40. _Sequence of Beds._--It requires little reflection to see that the division plane between two beds may represent a very long period of time. Let the following diagram represent a section of strata, _s_ being beds of grit, and _a_, _b_, _c_, beds of sandstone and shale. It is evident that the beds s must have been formed before the strata _b_ were deposited above them. At ×, the beds _a_ and _b_ come together, and were attention to be confined to that part of the section, the observer might be led to infer that no great space of time elapsed between the deposition of these two beds. Yet we see that an interval sufficient to allow of the formation of the beds _s_ must really have intervened. It is now well known that in many cases planes of bedding represent 'breaks in the succession' of strata--'breaks' which are often the equivalents of considerable thicknesses of strata. In one place, for example, we may have an apparently complete sequence of beds, as _a_, _b_, _c_, which a more extended knowledge of the same beds, as these are developed in some other locality, enables us to supplement, as _a_, _s_, _b_, _c_.