STRUCTURE AXD POSITION. 181 



amount of erosion. 4. A period of subsidence, during which the land- 

 surface became sea-bottom We can not estimate the length of this. 

 5. A long period of quiet, during which the second series of sediments 

 was continuously deposited. This period is estimated by the thickness 

 of the sediments. 6. Another period of elevation by which the whole 

 was brought into vieAV. The process is more fully explained in connec- 

 tion with a concrete example on page 294 of Part III. 



It is evident, then, that every case of unconformity represents a gap 

 in the geological record at that place ; for the geological record is written 

 on strata, and unconformity means a land-surface period, during which 

 there was erosion instead of sedimentation, record-destroying instead of 

 record-making. The gap may be filled and the record recovered by 

 sediments formed at that time in some other place. This is usually 

 the case, but not always. The loss of record may be partly by erosion, 

 but mostly because not written at that place. 



2sow, such unconformities and lost records are, as we have seen, the 

 result of crust oscillations. But crust oscillations produce necessarily 

 changes in physical geography, and therefore changes of climate, and 

 therefore also changes of faunas and floras. They consequently mark 

 the great divisions and subdivisions of geological history. 



Geological Formations. — A group of strata conformable throughout 

 and containing continuous record, and separated from other con- 

 formable groups by a line of unconformity, is called a geological for- 

 mation. There are, however, other tests of a formation, by which 

 hereafter we will complete the definition, 



Cleavage Structure* 



We have thus far spoken only of the original and universal structure 

 of stratified rocks, together with the tiltings, foldings, and erosion, to 

 which they have been subjected. There is, however, often found in 

 stratified rocks a superinduced structure which simulates, and is often 

 mistaken for stratification. It is called cleavage structure, or (since it 

 is usually found in slates) slaty cleavage. This subject has recently 

 attracted much attention, and is an admirable example of the successful 

 application of physics to the solution of problems in geology. 



Cleavage may be defined as the easy splitting of any substance in 

 planes parallel to each other. Such definite splitting may result, in 

 different cases, from entirely different causes. For example («), under 

 the influence of the sorting power of water, sedimentary materials may 

 be so arranged as to give rise to easy splitting along the planes of lami- 



* This structure is usually treated under metamorphic rocks, as a kind of metamor- 

 phism; but it is found in rocks which have not undergone ordinary metamorphic changes, 

 and it is produced by an entirely different cause. 



