MACHUGA FORMATION 313 



although a careful search was made, the thickest seam observed was under 

 half an inch. While sufficient carbonaceous matter is sometimes dis- 

 seminated through the shales to give them a black color, no indications 

 were found pointing to the existence of workable coal deposits in the 

 region examined. 



Coal in workable quantity has been reported from the region south- 

 west of the lake, between the lake shore and the Costa Rican volcanoes. 

 The exact locality is on the Rio Hacienda, 12 miles from its mouth. It 

 was not visited and no samples of the coal were seen, so that the report 

 lacks verification. There appears to be no reason, however, why condi- 

 tions favorable for coal accumulation should not have prevailed in some 

 portions of this region during the deposition of the Brito beds. 



MACHUCA FORMATION 



Distribution. — The immediate margins of the San Juan valley from 

 the lake eastward to Castillo are, so far as known, composed entirely of 

 igneous rocks. From a point a few miles below Castillo to another mid- 

 way between Machuca and the Boca San Carlos the rocks are largely 

 sedimentary, although they contain some igneous rocks in the form of 

 small dikes. These sedimentary rocks constitute the Machuca forma- 

 tion. Its present extent is known only in the immediate vicinity of the 

 river. The region south of the upper San Juan, forming the lower 

 valleys of the Frio and Poco Sol, is geologically unexplored. It is there- 

 fore possible that the Machuca formation may extend westward through 

 this region and be nearly, if not quite, continuous with the outcrops of 

 the Brito formation south of the lake. Until this connection is estab- 

 lished, however, the original continuity of the two formations is a matter 

 of doubt. 



Lithologic character. — The rock exposures in this region to the eastward 

 of Castillo are very much less satisfactory than those along the Pacific 

 coast ; hence the character of the Machuca formation is not so well 

 known as is that of the Brito. Like the latter, it apj)ears to consist 

 chiefly of calcareous shales, with which sandstones are interbedded. 

 The constituents of the rocks are largely igneous in their origin, but 

 there are no coarse conglomerates or breccias such as occur in the Brito. 



No pure limestones or distinctly marly beds have been discovered, 

 although the examination of the formation has not been sufficiently ex- 

 haustive to enable one to say that such beds do not occur. 



Structure. — The exposures are comparatively few in which the dip of 

 the Machuca sandstones can be determined. At the Cano Bartola the 

 dip is about 15 degrees and to the north. At Machuca it is 20 degrees 



