HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 233 



mountain gorges. Two miles beyond this station massive igneous 

 rocks again appear in the cuts beneath the debris and the immense 

 bluff of rounded bouldei-s, like those at Mata Chin upon the Isthmus 

 of Panama. ^Near Peralto there is a wide flat valley in the foot-hill 

 scenery, which, from a cutting, is seen to be made up almost entirely of 

 these boulders. Five miles east of Peralto there is a long tunnel 

 through an immense hill of rounded boulder conglomerate. Seven and 

 a half miles east of Peralto the railway reaches the river valley at an 

 altitude of 1,500 feet. The sedimentary Tertiary rocks are seen in the 

 bluffs on the opposite side of the river. At Pascaua another intrusive 

 neck of massive igneous material projects through these Tertiary sedi- 

 mentaries, which Professor Wolff determined to be Theralite. 1 



We now enter the lower hilly country which lies on the Caribbean 

 slope of the volcanic highlands of Costa Rica. This country topographi- 

 cally resembles the surface of the Isthmus ; it shows the same pointed 

 character of the hills, the same deeply eroded valleys, and the same 

 vegetation. One mile east of the station last mentioned, at an altitude of 

 880 feet, 150 feet of gi-eensand marls very much resembling the Mindi 

 beds of the Isthmian section are exposed in a cut. Two miles to the 

 east the river again cuts sedimentary beds dipping to the eastward. 



At Los Lomas, altitude 740 feet, there are great bluffs of Tertiary 

 greensand, the strata of which have a very strong dip of nearly 45 

 degrees. Massive rocks project through the sedimentaries. The river 

 is still in a canon 1,000 to 800 feet deep. Just before reaching Lajunta 

 the river cuts through a tuff resembling that of Bujio on the Isthmus. 

 At Lajunta we are apparently out of the mountain passes, but the great 

 bluffs of rounded boulder debris which have rolled down the slopes of 

 the volcanic plateaus for ages seem to cover this lower country. 



At Siquieres, altitude 390 feet, 37 miles from Limon, the hills are low, 

 not averaging over 200 feet in height, but the boulder bluffs continue in 

 great profusion. 



In Cartago, at the house of Mr. Jones, the station agent, we saw a 

 small specimen of white granite, which he informed me he had found up 

 the Eio Siquieres, and had broken off of an immense boulder which lay 

 in the river at that place. This stream is a small river which drains the 

 eastern foot of the Atlantic escarpment of the Turialba volcano, and 

 there is no reason to doubt that granitic rocks occur at the base of 

 this great concealing mass of igneous material. That such granites have 

 existed beneath the Costa Rican volcanic complex is still further attested 



1 See American Journal of Science, Vol. I. p. 291. 1896. 



