RECENT GEOLOGICAL HISTORY 331 



Overlying the shales of the Brito formation, there is a zone of weath- 

 ered rock which corresponds in some measure with the zone of soft rock 

 generally observed in the eastern division. Within this zone the beds 

 are thoroughly shattered by the presence of numerous joint planes, and 

 concentric weathering has been more or less extensively developed. 

 The mechanical alterations which the rocks have suffered, however, are 

 much more important and striking than the chemical changes ; hence 

 in the sections this is called the zone of disintegrated rather than weath- 

 ered rock. 



Recent geological History 



relationship between topography and geology 



The relation between the topography and the recent geological history 

 of the region is so intimate that a description of the former necessarily 

 involves some statements concerning the latter. The same is to a some- 

 what less extent true of the lithology ; hence in the foregoing descrip- 

 tion of the topography and of the rock formations some of the main fea- 

 tures of the geological history have been briefly outlined. With these 

 prerequisite facts of topography and lithology, the geological history may 

 now be taken up systematically and in some detail. 



CONDITIONS ANTERIOR TO TERTIARY TIME 



As already indicated, no rocks older than the Tertiary occur in the 

 region of the Nicaraguan depression, so that there is only negative evi- 

 dence as to the conditions which prevailed here during geological periods 

 earlier than the Tertiary. 



In the region to the northward in northern Nicaragua the occurrence 

 of granites and crystalline schists has been described ; also small areas 

 of Paleozoic rocks. The present extent of these older formations, how- 

 ever, as well as their former distribution, is not known. 



The region to the south in Costa Rica also contains older formations, 

 but they are almost completely covered by the recent volcanic rocks, so 

 that the former extent of the land in this direction also is unknown. It 

 is quite possible that a depression of this portion of the isthmus occurred 

 at the beginning of Tertiary time, and that a somewhat extensive land 

 area was wholly submerged or converted into an archipelago. 



EARLY TERTIARY DEPOSITION AND VOLCANIC ACTIVITY 



As indicated in the description of the Brito and Machuca formations, 

 these rocks were deposited on the sea bottom in early Tertiary time. It 

 is assumed that during their deposition there was open communication 



