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 cafion of the Rio 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 Miraflores Pumice. — At Miraflores in the bluffs bordering the 
valley of the swamp level plain (Colon, 41.69 miles) there is a clean cut 
Fieurn 11. Section at Miraflores, 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 
