STRUCTUEE OF THE BRITO FORMATION 311 



also conglomerates and breccias similar to those occurring at various 

 points along the Pacific coast. 



Structure. — The Brito formation, wherever observed, was found to be 

 intersected by numerous joint planes. In some places these occur as 

 two well developed sets of approximately parallel planes, which inter- 

 sect each other nearly at right angles. In others the joint planes are 

 very numerous and irregular, cutting the beds in all directions. The 

 latter form is less common and appears to be confined to rather narrow 

 zones where shearing and faulting has probably taken place. The fre- 

 quency of the joint planes varies with the thickness of the beds. The 

 rhomboidal blocks into which the beds of shale and sandstone are broken 

 usually have diameters approximately equal — that is, the more massive 

 the original beds the farther apart are the intersecting joint planes. 

 These joints have permitted the percolations of surface waters to great 

 depths and have facilitated the deep weathering which is generally ob- 

 served. The weathering proceeding outward from the joints has resulted 

 in the formation of concentric layers about a core, which coincides with 

 the center of the original rhomboidal block. The resulting rounded 

 blocks in some places give the appearance of a rude rubble wall. In the 

 vicinity of Las Lajas the horizontal sandstone beds have been lain bare 

 by the action of the waves, and where the rhomboidal blocks produced 

 by jointing have been rounded by concentric weathering the appearance 

 is that of a cobble pavement. 



The Brito formation has suffered only a moderate amount of disturb- 

 ance since its beds were deposited. Where its rocks are best exposed 

 along the Pacific coast, numerous small faults are observed, the displace- 

 ment in most cases being but fi few inches. The inclination of the beds 

 is generally under 20 degrees, though in a tew localities the disturbance 

 has been much greater and the dips increase up to the vertical. Neglect- 

 ing these minor irregularities, the dominant structure is a broad anticline 

 whose axis extends in a northwest-southeast direction approximately 

 parallel with the Pacific and lake shores and a short distance southwest 

 of the latter, where the beds are approximately horizontal. The greater 

 portion of the region between the lake and the Pacific, therefore, is occu- 

 pied by the western limb of the anticline and has prevailing southwest 

 dips. From San Jorge to Lajas the dips are somewhat variable, but gen- 

 erally to the northeast. The greater part of the eastern limb of the anti- 

 cline is covered by the lake. The numerous exposures of the Brito 

 formation along the southern margin of the lake from the Sapoa to the 

 Rio Orosi belong to this eastern limb of the anticline, and the beds have 

 northeasterly dips of 5 to 30 degrees. The strike of these beds is not 

 strictly parallel with that of the beds on the Pacific coast. They con- 



