384 STUDIES FOR STUDENTS 



only, but ultimately great changes take place separating earth history 

 into its periods and eras and giving to each an individual character. 



Marine conglomerates and sandstones, but especially conglomer- 

 ates restricted to those whose material is obtained and sorted by the 

 waves and transported by bottom currents, where widely developed 

 and intercalated between unlike formations, are indicative of broad 

 movements of the beach line ; that is, of the changing relations of land 

 and sea. 



Tectonic conglomerates and sandstones are of subaerial origin and 

 result from vertical earth movements, ultimately from either horizontal 

 or vertical forces. To separate them sharply from climatic con- 

 glomerates and sandstones, the climate is supposed to be unchanging 

 during the progress of the following erosion. 



Climatic conglomerates and sandstones are also of subaerial and 

 fiuviatile origin, but owe their contrasts with the superior and inferior 

 formations to chmatic and not tectonic changes. To separate them 

 clearly from deposits of tectonic origin earth movements must be sup- 

 posed quiescent while climatic variations of greater or less degree are 

 supposed to occur, resulting in changes of the sedimentary fades and 

 in shiftings of the regions of deposit of that land waste which arises 

 primarily through the contest of tectonic and atmospheric forces. 

 Where the climatic changes have been great and rapid, the nature of 

 the erosion may be so changed and the regions of deposit so widely 

 shifted that these climatic variations may be the cause of the most 

 striking differences between formations. Climatic conglomerates and 

 sandstones are here made distinct and independent from those of tec- 

 tonic origin by the taxonomic elevation of the shifting location of 

 deposits (in space) to co-ordinate importance with intermittent uplift 

 and resulting pulses of erosion (in time) . 



Changes in volume of ocean waters, earth movements, and atmos- 

 pheric activities are the three mixed and fundamental causes by which 

 the three classes of deposits become possible, but the records which 

 they embody are largely distinct and independent. By separating 

 conglomerates and sandstones into these three classes the sedimentary 

 rocks, therefore, present a threefold record, the marine conglomerates 

 giving that of the variable relations of land and sea; the tectonic 

 conglomerates, the record of variable vertical uplifts; the climatic 

 conglomerates, the record of variable temperature and rainfall. 



