10 PALAEONTOLOGY OF KENTUCKY. 



produced greater denudation. Then, again, other periods set in, when the 

 temperature of our globe was very low, when the excessive heat of former 

 ages was replaced by excessive cold. Frost is a very powerful agent in the 

 destruction of rocks. The hardest material which admits water into its pores 

 will be crumbled to dust by this destructive force. Here, again, denudation 

 must have been very large. Between these extremes in temperature, our planet 

 experienced some moderate climate which had not such a destructive influence 

 upon the solid material of our earth's crust. 



STKATA AND LAYERS. These two terms are generally used as synonymous, 

 but some geologists make a distinction between them. They use the term 

 layer for each single member of stratified rock, and apply the term strata to 

 beds of the same material. Thus, if a section shows in its lower half limestone, 

 and in its upper half sandstone, it contains only two strata, though it may 

 show a great many layers. 



Layers and strata vary in thickness from an inch, and less, to many feet. 

 Very thin layers are called laminae, and for thin strata, the term seams is used. 

 If all the strata had remained undisturbed in their original position, all would 

 be nearly horizontal, and parallel to each other ; but the many upheavals and 

 depressions to which our earth's crust has been subjected, have disturbed their 

 original horizontal and parallel arrangement, and we now find them occupying 

 every imaginable position in relation to each other. Wherever two sets of 

 strata or layers are nearly parallel to each other, it proves that the older set 

 was not disturbed in its original position before the younger or later one was 

 deposited upon it ; but whenever the upper strata rests on the edges of those 

 below, these latter have been disturbed before the formation of the upper 

 ones took place. In the first case, where the parallelism of the strata is main- 

 tained, they are said to be conformable; but whenever the planes of the upper 

 layers rest on the edges of the lower ones, they are called unconformable. 

 Conformability of strata indicates a period of rest, whilst unconformability is 

 a certain proof of disturbance.- Any movement in the earth's crust must pro- 

 duce some changes in its layers or strata, resulting either in a bending or 

 breaking of the same, whereby fissures, folds and faults are originated. Fis- 

 sures are rents caused by breakage, without any displacement of the rock on 

 either side of the fracture, below or above their former level ; but, whenever 

 the masses on one side or the other have changed their positions, either by 

 elevation or depression, the rent becomes a fault. According to the great 

 difference in the magnitude of the forces producing the faults, the size of the 

 latter must also differ greatly. We find them measuring from an inch and 

 less, to many hundred feet. Faults are of great inconvenience to miners, 

 especially where they appear of considerable size. Folds of strata are the 

 result of their bending without breaking ; they differ in size from a few feet 



