522 SIDNEY POWERS 
streams are now cutting narrow channels, and in the terraces along 
the continental shelf. Coast charts show a shelf about 5 miles 
wide with terraces at depths of about 15 and 30 fathoms in the 
vicinity of Utilla Island. The antecedent stream, Rio Dulce, 
Guatemala, flows through a rock gorge, and the depth of the water 
through the gorge and directly behind the bar is 4o feet. No other 
stream along the coast has a rock bed at sea-level. Therefore data 
on subsidence are very fragmentary. 
TECTONICS 
Guatemala and Honduras are composed of two mountain 
systems (Fig. 1): the Pacific Cordillera, now little more than a belt 
of high plateaus covered with young volcanics, and the Caribbean 
Cordillera, consisting of east-west ranges. On the north the tec- 
tonic lines of Yucatan trend toward the Isle of Pines; in the center 
of Guatemala they trend toward a 3,o00-fathom deep; on the 
south, in Honduras, they trend toward Jamaica. The two systems 
are in striking contrast; from the Atlantic, broad, structural valleys 
with a maximum length of 200 miles (Fig. 3) stretch almost across 
the Isthmus; from the Pacific, short, precipitous valleys extend to 
the high plateau between the volcanoes and almost disappear on the 
low plain near the coast. 
It has been pointed out above that successively older formations 
appear on the Caribbean side of the Isthmus from north to south 
and that the Tertiary and younger volcanics are largely confined to 
the high plateau forming the backbone of the Isthmus. Pleistocene 
volcanoes are aligned on the inland edge of the Pacific plain parallel 
to the coast. The lack of any evidence on the Pacific side of 
Guatemala of valleys of such size that they would not be concealed 
by late Tertiary and more recent volcanics, the absence of a coastal 
plain showing uplift, and the remarkable alignment of the volcanoes 
point to a possible fracture zone on the western side of which a 
portion of the Pacific Cordillera has subsided. Along one‘of the 
principal lines of subsidence at the intersection of cross-fractures 
the volcanoes have been built.t | Uniformly great depths in the 
«The fracturing is similar to, but more complicated than, that in the Hawaiian 
Islands (S. Powers, ‘‘ Tectonic Lines in the Hawaiian Islands,”’ Ball. Geol. Soc. Amer., 
XXVIII (1917), 501-14). 
