Page:A Treatise on Geology, volume 1.djvu/68

 52 commonly occurs. But when the strikes and dips of either the upper or lower set vary, which is a very common case, the same district, as Yorkshire or Derbyshire, may exhibit in the same region both conformity and unconformity between the same two sets of beds, as in the



map diagram (fig. 17); where a b c d are coal strata, with variable dips and strikes, generally unconfirmed, but on the line W E, for a short distance, conformed to the magnesian strata lying upon them (e f g h). Yet, in this case, the section on the line W E would exhibit a want of conformity in the dip (as in fig. 16.), the beds a b c d being more inclined than e f g h. When the strata are not in contact, or, for any reason the junction cannot be clearly seen, many observations of dip and strike in each set of beds will in general determine the existence of unconformity: but it would be folly to rest so important a decision upon testimony less demonstrative than the country will yield; and, in some cases, sufficient evidence is unattainable by any exertion of industry and skill.