THE NORTHERN MARGIN OP THE DEPRESSION 299 



destroyed all trace of the original constructional topography, and the 

 location of the vents by which the volcanic rocks were erupted can not 

 be determined from the present form of the surface, though it might be 

 determined by a systematic study of the distribution and variations in 

 character of the volcanic rocks. Towai'd the northern end of the lake, 

 opposite Granada, the summits of the hills present an even skyline, as 

 though they were remnants of a plateau ; but this surface may be a 

 degradational rather than a constructional plain. The streams flowing 

 into the lake have baseleveled their valleys for a considerable distance 

 back into the upland, but are separated by sharp ridges and hills which 

 occupy the divides. Although the higher portions of the divides attain 

 somewhat uniform altitudes which increase northward, the uniformity 

 is not sufficient to determine the former existence of a distinct plain, 

 and it is probable that the present valleys are carved in a surface which, 

 since its final emergence above scale vel, has always had rather high relief 



Western divide. — As already indicated, the great Nicaraguan depression 

 was formed before lake Nicaragua came into existence. It originally 

 extended entirely across the isthmus, terminating to the westward at 

 the bay which then indented the Pacific coast, a cape projecting to the 

 northwest between this bay and the ocean. The cape now forms the 

 narrow strip of land lying to the southwestward of lake Nicaragua and 

 separating it from the Pacific. This strip of land is not proper]}^ there- 

 fore, a part of the Nicaraguan depression, and its topography should be 

 independently considered. 



Bordering the southwestern shore of the lake and extending north- 

 westward nearly to Zapetara island is a ver}^ perfectly baseleveled sur- 

 face, termed for convenience the Rivas plain. It varies in width from 

 5 to 12 miles, and is continuous along the lake margin, except near the 

 Sapoa river, where it is interrupted for a short distance by high hills 

 coming down to the lake. Very little is known concerning the south- 

 eastern extension of this plain, but it is probably nearly or quite con- 

 tinuous around the end of the lake with the peneplain of the Nicaraguan 

 depression already described. Its northeastern margin is the lakeshore, 

 where the waves have cut a shallow terrace backed by a cliflf from 10 to 

 40 feet in higlit. A few low, rounded hills rise above its even surface, 

 but they seldom attain hights of more than 100 feet. In the .vicinity of 

 Rivas, where it is most thoroughly known, the plain ascends toward the 

 southwest, at the rate of about 8 feet to the mile, to the base of the hills 

 which occupy the greater part of this strip and form the Continental 

 divide. These hills rise abruptly from the Rivas plain to hights of 800 

 to 1,200 feet above tide, and extend northward to a point opposite the 

 island of Zapatera, where they meet the Jinotepe plateau, and the ser- 



