508 O. H. HERSHEY GEOLOGICAL RECONNAISSANCE IN NICARAGUA 



sending long arms far into the delta country. It is typically developed 

 in the bluff at Saclin. The character of its distribution points toward a 

 marine origin. The pebbles are predominantly white quartz, probably 

 largely derived from the gneissic and other old formations described by 

 Belt and Crawford from the headwaters of the Wanks Eiver, although 

 they adjoin a region predominantly igneous. I attribute this to the 

 action of the sea in wearing out the softer igneous rocks. The quartz 

 gravel of the Lower Wanks probably comes largely from erosion of the 

 Saclin formation. 



In short, the "Pine Eidge" country is probably an old dissected coastal 

 plain. The old river channels mentioned by Crawford probably emerge 

 from the mountains at the same level. It may also be fairly safe to cor- 

 relate this coastal plain with the peneplain discriminated by Hayes in 

 the "ISTicaraguan depression." In age the Saclin formation is probably 

 rather early Quaternary. 



Tertiary Eocks 

 in general 



Being in the great Atlantic forest, with an average annual precipita- 

 tion of about 138 inches, a minimum temperature at the Bonanza mine 

 of 52 degrees Fahrenheit and a common daily range of 60 degrees Fah- 

 renheit to 90 degrees Fahrenheit or 96 degrees Fahrenheit and a dense 

 vegetation, the rocks of the Pis-Pis district are near the surface most 

 thoroughly decomposed and the areal geology extremely difficult to study 

 except along the larger creeks, in the mine excavations, and at wide 

 intervals in the trails. The zone of oxidation probably averages 100 feet 

 in depth on the hills. Where the soil is stony, the rock fragments are 

 almost exclusively the debris from quartz veins. Accurate detailed map- 

 ping is impossible except in the vicinity of the best developed mines. 

 Furthermore, it was difficult to secure fresh material for microscopic 

 study. A set of tliin-sections was submitted to Dr. A. C. Lawson, who 

 has kindly furnished me the descriptive notes printed herewith in small 

 type. They were made by one of the students at the University of Cali- 

 fornia, but reviewed and amended by Doctor Lawson, who is responsible 

 for the names that I am going to apply to the rocks. 



ANDESITE 



The larger part of the district is occupied by an extrusive series of 

 andesitic composition. Its structure is generally obscure, but frag- 



