324 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



even as in the eastern division, at very great depths heneath tlie thick 

 sheet of dacite, they are found in the form of a soft, soapy or talcose rock. 



Tiie rocks of the Brito formation contain a much smaller pro2)ortion 

 of igneous constituents than any of those in the eastern division ; hence 

 they are in a measure free from this source of weakness. They contain, 

 however, a large proportion of lime carbonate, and in them the weather- 

 ing process consists chiefly in the solution and leaching out of this 

 cementing material. In most of these rocks the lime forms so small a 

 proportion of the entire mass that the bulk is not diminished or the 

 structure altered by its removal. The rock merely changes in color from 

 bluish gra}^ to brown or yellow, and at the same time it becomes soft 

 and porous. 



Effect of original structure. — The original structure of many of the rocks 

 is such as to facilitate weathering to a considerable degree. This is 

 especially true of the basalts, which are largely composed of surface lava 

 flows and have the vesicular structure which is characteristic of such 

 flows. 



In many cases it is observed that the degree of weathering in the case 

 of basalts varies directl}^ with the extent of the vesicular structure. The 

 upper and lower surfaces of the flows which were rapidly cooled by con- 

 tact with the underlying rocks and by exposure to the air contain more 

 or less abundant gas bubbles, while their central portions are relatively 

 compact. In such cases it is found that the vesicular portions are thor- 

 oughly weathered, while the interior compact portion contains large 

 boulders of fresh rock or continuous beds of the same. The dacite, 

 which, so far as observed, never has the vesicular structure of the basalt, 

 does not show these striking differences in the degree to which its differ- 

 ent portions have weathered. 



The depth of weathering in the volcanic sandstones and conglomerates 

 naturally depends largely on the original structure of their constituents, 

 which shows considerable variation. Thus the conglomerate encoun- 

 tered at upper Ochoa is composed chiefly of pebbles of compact, fine 

 grained basalt, and is weathered only to a moderate depth. A con- 

 glomerate was encountered at lower Ochoa similar to the above, except 

 that its constituent pebbles are largely composed of vesicular or i)umi- 

 cious basalt. This difference in the composition of the pebbles is accom- 

 panied by a corresponding difference in the depth of weathering, which 

 has extended to a very great depth in case of the latter rock. 



Effect of secondary structures. — A third important factor in determining 

 the depth to which rock decay has gone is the extent to which the rocks 

 have been affected by dynamic agencies with the production of secondary 

 structures, such as folds, faults, and joint planes. Of these effects joint- 



