CLASSIFICATION OF THE ROCKS 309 



hills in their broad meanders. The only exception to this general state- 

 ment is the San Juan river between Castillo and Machuca. The con- 

 ditions west of lake Nicaragua are somewhat more favorable. The 

 vegetation is not so abundant, and the removal of the residual matter 

 has more nearly kejDt pace with the rock decay ; also the slopes are 

 more abrupt, and most of the streams are corrading their channels, 

 except in the lower portions of their valleys. 



CLASSIFICATION OF THE BOCKS 



The rocks of the region are placed in two groups — Tertiary and post- 

 Tertiary. Each includes both igneous and sedimentary formations. No 

 rocks certainly older than the Tertiary occur along the line of the canal, 

 although such have been reported from northern Nicaragua and also from 

 central Costa Rica. The Tertiary sedimentary formations include the 

 Brito and Machuca. 



BRITO FORMATION 



Distribution. — With the exception of a few areas of intrusive igneous 

 rocks, the strip of land between lake Nicaragua and the Pacific is occu- 

 pied entirely by the Brito sandstone. It extends from the Sapoa river 

 to a point opposite the island of Zapatera. Remnants of the formation 

 are also found along the lake shore to the southeast of Sapoa, and its 

 present outcrops may extend continuously eastward to the area occu- 

 pied by the Machuca sandstone. To the southward the formation is 

 probably covered by the recent lavas of the Costa Rican volcanoes. It 

 also probably extends some distance to the northwest of Zapatera, where 

 it is covered by the recent tuffs which form the Jinotepe plateau. 



Lithologic character. — The formation presents considerable variety in 

 its lithological composition, but it has not yet been sufficiently studied 

 to permit of its subdivision, even if this may be eventually possible. 

 Much the larger mass of the formation consists of somewhat calcareous 

 non-fissile shale. When fresh this is bluish gray and weathers to a yel- 

 lowish or brownish color. 



Distributed through the shale are numerous beds of sandstone. These 

 are also somewhat calcareous and doubtless contain a considerable pro- 

 portion of volcanic ash. The sandstone beds vary in thickness from a 

 few inches to two or three feet, and occur singly or in groups. The 

 latter are sufficiently heavy to materially affect the topography in some 

 places. These sandstones, like the shales, are blue when entirely fresh, 

 but are alwa3^s weathered at the surface to some shade of yellow or brown. 

 The hills immediately west of Rivas, rising abruptly above the Rivas plain, 

 are due chiefly to the presence of these resistant sandstones. They occur 



XLV— Bum,. Geoi.. Soc. Am., Vol. 10, 1898 



