January 27, 1899.] 



SCIENCE. 



153 



These uneroded tables, however, have a faint 

 topography due to subsidence. The flat surface 

 is extensively pitted with saucer-form depres- 

 sions. Their dimensions vary from a foot or 

 two in depth and a breadth of 100 feet to 75 

 feet in depth and a breadth of two or three 

 miles. Wind action is plainly to be excluded. 

 Occasionally they are surrounded, upon the 

 hard, sod flats, by concentric cracks, deep 

 enough to cause injury to cattle and to entrap 

 calves. 



These forms, large and small, without differ- 

 ences in type, are attributed to the operation of 

 two distinct and unrelated processes : (1) set- 

 tlement and compacting within the deep and 

 unconsolidated mass of silt sand and gravel, 

 through instrumentality of the ground-water ; 

 and (2) solution of salt and gypsum, and conse- 

 quent caving, within the rocks of the floor, 

 where those rocks are the Red Beds. 



The concurrence of two unusual causes, to 

 produce, alike, within a limited area, a result 

 so unusual, appears, however, to be intelligible 

 on this assumption, viz.: These processes are, 

 in fact, of universal operation ; they are at the 

 same time of too light effect to stand a chance 

 in competition with erosive agency ; but the 

 sod-covered, subhumid plains, remarkable in 

 that they retain a flat surface unscored by 

 erosion, aiford exceptional opportunity for the 

 preservation of their delicate record. 



The Lake Region in Central America. — The 

 region described includes southern Nicaragua 

 and northern Costa Rica, extending from 10 

 degrees and 30 minutes to 12 degrees and 30 

 minutes north latitude, and from the Carribean 

 to the Pacific. It includes the route of the pro- 

 jected Nicaragua Canal and the largest lakes of 

 the western hemisphere south of the glaciated 

 region of North America. The region is char- 

 acterized by two types of topography, viz. : (1) 

 the recent volcanic ranges and plateaus in which 

 the original constructional forms are more or 

 less perfectly preserved ; and (2) the areas of 

 Tertiary, igneous and sedimentary rocks in 

 which the drainage is mature and the forms are 

 due to long continued subaerial erosion. A note- 

 worthy feature is the absence of any continuous 

 mountain range or chain of dominant peaks 

 through this portion of the isthmus. A shallow 



depression occupies the western portion of the 

 region, its longer axis being nearly parallel with 

 the Pacific coast. This contains Lakes Nica- 

 ragua and Managua. The former is 110 miles 

 in length, with an area of 3,000 square miles 

 and a mean altitude of 106 feet. Its greatest 

 depth is 200 feet. 



The climate of the region is tropical and in- 

 sular, the annual range of temperature being 

 small. The rainfall is greatest on the east coast, 

 nearly 300 inches at Greytown, and decreases 

 somewhat uniformly westward, being less than 

 80 inches on the west coast. Connected with 

 the decrease in the rainfall there is a striking 

 change in the character of the vegetation, the 

 dense tropical jungle of the east coast giving 

 place to open forests and savannahs in the west. 



No rocks older than the Tertiary are formed 

 along the line of the canal. They consist of 

 eruptive and sedimentary formations, the former 

 including basalt, andesite and dacite, and the 

 latter calcarious sandstones and shales. In ad- 

 dition to these Tertiary rocks there are exten- 

 sive recent alluvial deposits and the tuffs and 

 lavas of the modern volcanoes. The conditions 

 throughout the region, 'hnt particularly in its 

 eastern portion, are favorable for rock decay, 

 and the regolith is unusually extensive. 



The late geologic history of the region is 

 briefly as follows : In early Tertiary time this 

 portion of the isthmus may have been wholly 

 submerged. At any rate, marine sediments 

 were deposited throughout a considerable part 

 of its extent, and this was accompanied by 

 intense volcanic activity. In middle Tertiary 

 time there was an uplift and long continued 

 erosion, the constructional volcanic topography 

 being obliterated, and the region, at least 

 toward the south, being reduced to one of low 

 relief. The present basin of Lake Nicaragua 

 was then occupied in part by a gulf connected 

 with the Pacific to the northwest and in part by 

 the valleys of tributary streams. The conti- 

 nental divide then occupied the hilly or moun- 

 tainous region east of the lake, crossing the 

 present San Juan valley near Castillo. In late 

 Tertiary or post-Tertiary time the isthmus was 

 elevated at least 300 feet and deeply dissected. 

 Following the elevation was a renewal of vol- 

 canic activity. A series of vents opened on the 



