198 bulletin: museum of comparative zoology. 



Plate XIII.) The material is a dark colored tuff, with large enclosures 

 of angular and rounded fragments. From this point on, these rocks 

 are well exposed in the cuts of both the railway and canal, and in the 

 first headwaters canon of the Eio Grande (at Rio Grande Superior, 37.30 

 miles) about one kilometer south of Culebra Station, and thence most 

 of the way to Panama Station. 



In the deep cuttings near Rio Grande Superior (37.30 miles), this 

 tuff seems to lie above more massive basalts exposed in the great cut 

 made for diverting the railway from the route of the canal. The 

 summit of the hill shows a great thickness of residual (non-strati- 

 fied) clay. 



At Curacacha (38 miles) there are still other cuts in the massive 

 basalt, which continues to Paraiso, 39 miles. Thus it will be seen that 

 for a distance of two miles along the rougher, upper portion of the 

 descent to the Pacific extending from the summit pass to Paraiso, as 

 determined by Prof. Wolff's examination of the specimens, the railway 

 and canal run mostly through diabase, or coarse basalt, similar to the 

 summit of Culebra Mountain, together with alternations of dolerite, and 

 a " dirty gray tuff, containing fragments of glassy basalt." 



At Paraiso is seen the first evidence of a formation which, appearing 

 at various places along the Pacific coast, becomes of great importance in 

 the Isthmian story, and which will presently be described more fully. 

 This consists of an ancient looking whitish conglomerate. 



Near Pedro Miguel (40.34 miles) another outcrop of this material is 

 seen resting on the basalt. The road now reaches the interior end of 

 the swamp level plain which indents the rugged hills from Panama to 

 this point. 



The Mirajiores Pumice. — At Mirafiores in the bluffs bordering the 

 valley of the swamp level plain (Colon, 41.69 miles) there is a clean cut 



Figure 11. Section at Mirafiores, showing Disturbances. 



vertical section (see Fig. 10) through a hill of peculiar material. With 

 the exception of the conglomerates above mentioned this is the first out- 

 crop and the only clear exposure of apparently sedimentary rocks between 

 the Culebra Summit and Panama. The vertical face of the cliff is about 



