RECENT ALLUVIAL FORMATIONS 319 



practically horizontal. The planes separating lava flows generally have 

 a decided original inclination, and this may be increased or diminished 

 by subsequent tilting. In the sections of the upper and lower Ochoa 

 damsites (plate 31) the planes separating the several formations have a 

 slight dip to the northeast. The same thing is observed in the sections 

 of the San Francisco embankment line and of the Eastern divide. In 

 so far as these dips are due to deformation, they suggest the presence of 

 a low anticline to the east of the Machuca basin, its axis approximately 

 parallel with the Caribbean coast and crossing the San Juan near the 

 Boca San Carlos. For reasons given above the structure of these igneous 

 formations, as well as of the Machuca sandstone shown on the geolog- 

 ical sections (plate 32), rests on a very few observations and should not 

 be accepted with too great confidence. 



RECENT ALLU VIA L FORMA TIONS 



The post-Tertiary formations of the region include the recent deposits 

 which make up the floodplains of the rivers and the deltaplains about 

 their mouths, together with the products of the recent volcanic activity. 



The character of the alluvium has been somewhat fully described on 

 a previous page and requires but little further mention. It varies in 

 character with the local conditions under which it is deposited and with 

 the character of the rocks from which it is derived. On the west side, 

 filling the valley of the Rio Grande, it consists of fine brown sand and 

 clay, derived from the decay chiefly of the sandstones of the Brito for- 

 mation. In some places it contains enough calcareous cement, which 

 has been deposited by infiltration from above, to give the alluvium a 

 fair degree of coherence. 



In the valley of the San Juan there is considerably wider diversity in 

 the character of the alluvium. In the upper portion of the valley it 

 consists of fine blue clay interbedded with fine blue and brown sand. 

 The sand occurs chiefly in the river channel and is the residuum which 

 the sluggish current of the river has been unable to transport. It is 

 probable that but little sand would be encountered in the alluvium at 

 any considerable distance from the present channel. 



In the lower portion of the valley the alluvium in the immediate 

 vicinity of the river contains considerable black sand, such as it is at 

 present transporting in great volume. This occurs either disseminated 

 through the finer silt which is derived from the decay of rocks in the 

 adjoining region or it occurs as distinct layers interstratified with the 

 clay. The presence of a considerable proportion of sand in the silt ren- 

 ders it much firmer than when the latter consists chiefly of clay. The 



