710 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The greater part of the Brito formation is apparently barren of organic remains. The 

 only locations at which fossils have been found are on or near the Pacific coast. This, however, 

 may be due to the fact that the rock exposures are not elsewhere of such a character as to facili- 

 tate the discovery of fossils, and the latter may possibly be more generally distributed than 

 present knowledge would indicate. The fossils are confined almost wholly to the limestones 

 and marly beds. They consist of corals, moUuscans and foraminiferal remains. The latter 

 are especially abundant. The rather meager collections have been submitted to Dr. Dall for 

 determination. He pronounces them OUgocene and probably identical with the foraminiferal 

 beds described by HiU from the Caribbean coast at Panama. One of the most abundant 

 forms is a small nummuhte, Orhitoides, prohahlj forhesei, which is characteristic of the lower 

 Oligocene. The moUuscan remains were collected on the Pacific coast about 75 miles north- 

 west of Brito in what was supposed to be a higher portion of the same formation. Dr. DaU 

 states that these have the upper Oligocene , aspect, though there are not enough of them to 

 be conclusive. He thus confirms the view entertained in the field that successively higher beds 

 in the Brito formation are exposed along the coast toward the northwest. 



In addition to the fossils on which is based the above conclusion concerning the age of the 

 Brito formation, it also contains rather abundant plant remains. They are in the form of 

 driftwood and coal, and as yet no remains sufficiently well preserved for identification have been 

 discovered. Associated with the coarser sandstones are numerous blocks of wood, whose 

 rounded forms suggest that they are fragments of drift which were incorporated with the sand 

 and gravel while it was accumulating. In some cases they still contain a large proportion of 

 their original carbon, and in others this has been more or less perfectly replaced by silica or 

 iron pyrites. The coal occurs associated with the fiiner sediments, and although a careful search 

 was made, the thickest seam observed was under half an inch. While sufficient carbonaceous 

 matter is sometimes disseminated 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 southwest of the lake, 

 between the lake shore and the Costa Rican volcanoes. The exact locahty is on the Rio 

 Hacienda, 12 mUes 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 conditions 

 favorable for coal accumulation should not have prevailed in some portions of this region during 

 the deposition of the Brito beds. * * h: 



[The Machuca formation] appears to consist chiefly of calcareous shales with which sand- 

 stones 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. Also no pure lime- 

 stones or distinctly marly b^ds have been discovered, although the examination of the formation 

 has not been sufficiently exhaustive to enable one to say that such beds do not occur. * * * 



No fossils have as yet been found in the Machuca formation which are sufficiently well pre- 

 served for specific determination. At Cruzita, 1 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 weathering of the rock with the removal of the soluble limestone, 

 and they are also shown in the thin section under the microscope. While they can not be 

 identified, they strongly suggest the forms which occur so abundantly 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 may be determined, rests upon other and less satisfactory 

 evidence. It is believed to be nearly or quite contemporaneous with the Brito formation — 

 that is, Oligocene (Tertiary) . The grounds on which this conclusion is based are briefly as 

 follows: (1) 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 they were deposited. The value of this fact for correlation depends upon the 



