CONDITIONS OP TERTIARY DEPOSITION 333 



of the material of which the sedimentary rocks are composed, but it con- 

 tinued for some time after their deposition, and produced numerous 

 dikes, cutting the beds, and also extensive lava flows, which in places 

 rest upon them. This volcanic activity appears to have been much more 

 violent and long continued near the axis of the present isthmus than on 

 the west side. 



The region between lake Nicaragua and the Pacific ocean, as already 

 indicated, is occupied chiefly by sedimentary beds and b}'' recent volcanic 

 material. Only a few large areas and occasional dikes of intrusive rocks 

 have been found associated with the Brito formation, and it is not cer- 

 tain that these ever reached the surface. While the coarse conglomerates 

 along the Pacific coast demonstrate the near proximity of volcanoes, the 

 indications are that the volcanic vents from which this material was de- 

 rived were to the west of the present coastline. 



The conglomerates are confined, so far as known, to the immediate 

 margin of the ocean, and the source of the material seems clearly to 

 have been to the westward. The similar conglomerates which occur on 

 the soutliwest shore of the lake appear to have been derived from vents 

 to the southward, and to mark the southern margin of the sea in which 

 the Brito formation was deposited. 



As stated above, the Tertiary volcanic activity was most prevalent in 

 the region east of the lake. More than two-thirds of the area which has 

 been examined between the lake and the Caribbean is now occupied by 

 igneous rocks, which present considerable variety in composition and 

 structure. It is probable that the present area of the Machuca forma- 

 tion does not represent its original extent, but merely a region in which 

 the volcanic rocks have failed to wholly conceal the sediments. 



The numerous beds of conglomerate and stratified ash associated with 

 the lavas in the region eastward from Machuca point to the presence of 

 standing water during the period of volcanic activity. This water in 

 which the ejecta were deposited may have been a shallow sea, from whose 

 bed the volcanoes rose, or a series of lakes formed on the imperfectly 

 drained constructional surface. It is very difficult, however, to determine 

 even approximately the conditions which prevailed during the deposi- 

 tion of this heterogeneous collection of formations. The difficulty is 

 of course greatly enhanced by the deeply weathered condition in which 

 the rocks are now found. 



MIDDLE TERTIARY UPLIFT AND EROSION 



The period of deposition in this region appears to have been termi- 

 inated toward Middle Tertiary time by an uplift which was coincident 



XLVIII— BuLt. Geol. Soc. Am., Vol. 10, 1898 



