234 



SCIENCE. 



[N. S. Vol. XI. No. 267. 



and returned home, the Field Museum party 

 made a new stripping beside the old and ob- 

 tained from it large additions to its summer's 

 collection. Photographs of the fossils exposed 

 in the various stages of the work of this party 

 formed the subjects of many of the illustrations 

 used in the Cosmopolitan article. 



Elmeb S. Eiggs. • 

 Field Colusibian Museum. 

 January, 23, 1900. 



CURRENT NOTES ON PHYSIOGRAPHY. 



PHYSIOGRAPHY OF JAMAICA. 



In connection with his studies of coral reefs, 

 Mr. Alexander Agassiz has had surveys made 

 of several West Indian islands by R. T. Hill, 

 whose latest report is on the ' Geology and 

 Physical Geography of Jamaica ' (Bull. Museum 

 Comp. Zool., Harvard College, xxxiv, 1899, 

 266 p., xli pL, including a topographical and 

 a geological map). The island contains an 

 interior mountainous area (the Blue mountains, 

 7360 feet), of greatly deformed rocks and of 

 well subdued and elaborately carved form, oc- 

 cupying about one-sixth of the total area, chiefly 

 in the east. A limestone plateau, whose gently 

 arched strata were deposited unconformably 

 upon the denuded older rocks during a period 

 of submergence, rims around the eastern moun- 

 tains and covers the central and western parts 

 of the island to heights of 3000 feet ; it is ter- 

 minated toward the coast by strong blufls, often 

 terraced, 1200 feet high on the north. Below 

 the bluffs, low plains descend gently to the sea. 

 Solution has exerted great control over the 

 drainage of the plateau, as may be seen in the 

 interesting series of depressions, from incipient 

 hollows of small size, to deep ' cock-pits ' or 

 sink-holes, and great basins, walled in by strong 

 cliffs. Some of the basins still have under- 

 ground drainage ; others discharge their waters 

 through canyons that have been formed by the 

 retrogressive erosion of exterior rivers ; while 

 still others have lost their outer wall by the 

 greater advance of erosion, subaerial or marine, 

 and now form amphitheaters open to the coast. 

 Inliers of older rocks sometimes rise in moun- 

 tain form from the floor of the larger basin, as 

 in Clarendon valley in the center of the island. 

 The strata of the coastal plains lie on denuded 



benches of limestone or older rocks ; their sur- 

 face is diversified by coral reefs and transverse 

 valleys, the first deposited, the second eroded 

 during the time of elevation. The largest plain 

 is that of Liguanea, upon which Kingston is 

 situated. 



The geological structure and history of the 

 island, and its relation to the surrounding re- 

 gions are fully discussed. 



NICAEAGUA CANAL EOUTE. 



No article recently published gives better 

 illustration of the practical value of the ex- 

 planatory or genetic method in geographical 

 description than that by Hayes on the ' Phy- 

 siography of the Nicaragua Canal route ' (Nat. 

 Geogr. Mag., x, 1899, 233-246; see also 

 Science, x, 1899, 97-104). One feels on read- 

 ing it that the author has critically observed the 

 salient facts and that his account of them fully 

 expresses the results of his observations. The 

 region described may be divided into three 

 parts : The upland traversed by the San Juan 

 river from Lake Nicaragua eastward to the 

 Carribbean, the basin of the lake, and the up- 

 land or continental divide that separates tho 

 lake from the Pacific. The eastern upland is 

 part of an uplifted and dissected peneplain, 

 100-200 feet above sea level, and bordered by 

 hills and mountains on the north and south. 

 Its revived streams still run nearly at the up- 

 land level in their upper courses ; then they 

 descend rapidly in young valleys that they are 

 still deepening to aggraded alluvial floors, 

 which suggest a recent depression after the 

 time of first valley cutting. The lake now 

 stands where a bay once opened northwest to 

 the Pacific ; the eastern upland was then the 

 continental divide. The bay seems to have 

 been formed by warping or faulting a western 

 portion of the peneplain above referred to. 

 Numerous volcanic cones grew on the bay floor 

 and converted its head into the lake, whose 

 level rose until an eastern overflow formed the 

 San Juan river, now cutting a trench across the 

 eastern upland. The southwestern barrier of 

 the lake seems to be another part of the pene- 

 plain, warped so as to give a steep descent to the 

 Pacific and a gentler descent to the lake. On 

 account of the unequal slopes thus determined, 



