AGE OF MACHUCA FORMATION 315 



There is a general similarity in lithologic composition and appearance 

 between the two formations. (2) Both have suffered about the same 

 amount of deformation, elevation, and erosion since the}'' were deposited. 

 The value of this fact for correlation depends on the proximity of the 

 areas which they occupy and the evidence that the recent geologic con- 

 ditions have been similar in both. (3) Both formations bear about the 

 same relation to a group of igneous rocks, which was in part contempo- 

 rary with them and in part subsequently invaded their beds. The dif- 

 ferences in composition of these intrusive rocks are not greater tlian 

 differences in igneous rocks within the same area, which are known to 

 be nearly or quite contemporaneous. (4) Finally, as pointed out above, 

 it is quite possible and even probable that the two formations are nearly 

 or quite continuous through the southern part of the upper San Juan 

 valley. 



In the vicinity of the Toro rapids, some distance westward from the 

 present limit of the Machuca formation, a few siliceous boulders have 

 been found which contain fossil remains. The original location of the 

 beds from which these boulders are derived is not known, though they 

 have probably not been transported a great distance. These fossils are 

 unfortunately only casts. They have been examined by Dr Dall, who 

 says they " are not determinable, but have the general look of a fresh- 

 water assembly." They are not regarded, however, as having any special 

 bearing on the age of the Machuca, since it is by no means certain that 

 they have been derived from that formation. 



TERTIARY IGNEOUS ROCKS* 



Location and general character. — As stated above, the beds of the Ma- 

 chuca formation occupy a broad belt, which crosses the valley of the 

 San Juan, extending from a point a little below Castillo eastward some 

 distance beyond Machuca. While this formation contains a consider- 

 able proportion of volcanic material and is intersected by numerous 

 dikes, it contains no lava flows and no beds the constituents of which 

 are exclusively of volcanic origin. In the remainder of the country be- 

 tween the lake and the Caribbean, wherever the underlying rocks or 

 their residual products rise above the recent alluvium of the floodplains, 

 the rocks are almost entirely of volcanic origin. The}'' present a great 

 variety in structure and appearance, varying through the extreme types 

 of volcanic products from distinctly stratified beds of fine volcanic ash, 

 through well rounded conglomerates, fine and coarse angular breccias, 



* The writer is indebted to Dr F. L. Ransome, of the U. S. Geological Survey, for a petrographic 

 examination of the igneous rocks collected in Nicaragua and determination of the rock species. 



