314 C. W. PIAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



and to the northwest. These dips suggest the presence of a synclinal 

 basin, the southern end of which is crossed by the San Juan. They are 

 not sufficient, however, to locate its axis. Although in general the dips 

 are light, the formation has suffered considerable local disturbance. 

 Breccias, j)robably due to faulting, have been observed at several points, 

 the best example being the ledge which projects into the river opposite 

 the mouth of the Machuca ; also numerous sharp folds occur in the 

 vicinity of Machuca. The same evidence of faulting and folding would 

 probably be found elsewhere if the exposures were sufficiently abundant 

 to render the structure determinable. 



The rocks of the Machuca formation are generally found deeply weath- 

 ered. The weathering is hastened by the igneous constituents which 

 they contain, and the final product is a residual red clay, which is indis- 

 tinguishable from the product of the decay of igneous rocks. Except 

 for the fresh rock obtained beneath the residual mantle by means of the 

 diamond drill, it would have been impossible to determine even approx- 

 imately the limits of the sedimentary and igneous rocks. At some points, 

 as at Machuca, the sandstone contains a very large proportion of iron 

 p^^'ites, which by oxidation also tends to hasten its decay. 



Nearly ever^'where the beds are intersected by numerous joint planes, 

 the only marked exception being the rather massive interbedded sand- 

 stones exposed on Machuca creek. Weathering has proceeded inward 

 from the joints toward the centers of the rhomboidal blocks, producing 

 concentric shells about a central nucleus exactlj^as in the Brito formation. 



Age of the formation. — No fossils have yet been found in the Machuca 

 formation which are sufficiently well preserved for specific determination. 

 At Cruzita, one mile below Machuca, the core from the diamond-drill 

 hole in the bed of the river contains numerous indistinct organic forms. 

 The rock is described by Dr Ransome as an andesitic tuff containing 

 fragments of limestone. The organic forms are revealed by the weath- 

 ering of the rock with the removal of the soluble limestone, and the}'' are 

 also sliown in tlie thin-section under the microscope. Wliile they can 

 not be identified they strongly suggest the forms which occur so abun- 

 dantly in portions of the Brito formation. The beds in which they occur 

 are evidently derived in large part from fresh volcanic tuff, though the 

 latter was not so abundant as to prevent the growth of organisms in the 

 sea in which it was being deposited. 



In the absence of conclusive fossil evidence, therefore, the age of the 

 Machuca formation, so far as it ma}^ be determined, rests on other and 

 less satisfactory evidence. It is believed to be nearly or quite contem- 

 poraneous with the Brito formation — that is, Oligocene (Tertiary). The 

 grounds on which this conclusion is based are briefly as follows : (1) 



