120 GEOLOGY. 



worked at present. The gold and silver lodes form the principal 

 mineral resources of the territory. The average yield of the gold lodes 

 in the Central City district is reckoned at 3 oz. per ton of 2000 lbs. The 

 silver mines are in the Georgetown district. The lodes occur in gneiss, 

 granite, and syenite. The ores are true silver minerals, such as pyrar- 

 gyrite, mixed with galena, zinc blend, and highly argentiferous fahlerz. 



C. L. N. F. 



Frazer, Prof. Persifor. On the Trias of York County, Pa. Proc. Ac. 

 JSat. Sci. ndladd. p. 123. 



. On the Structure of the York County Valley Limestone, and 



on the Micro -photography of Minerals. Ihid. pp. 128, 129. 

 There is a fault along the S.E. haK of the valley, instead of a double 

 anticlinal. 



. Origin of the Lower Silurian Limonites of York and Adams 



Counties, Pennsylvania. Proc. Amer. Phil. JSoc. March ; abstract 



in Amer. Journ. ser. 3, vol. ix. p. 471. 

 Attributes the origin of the ore to the presence of the pyrites in the 

 slates. The beds usually lie between the slate and L. Silurian lime- 

 stone ; but the latter is generally little, if at all, acted upon or eroded, 

 and often is not even stained. G. A. L. 



G-abb, W. M. Notes on Costa Eica Geology. Amer. Journ. ser 3, 

 vol. ix. pp. 198-204. 



A resume of the geology of the district of Talamanca, the S.E. corner 

 of Costa Eica, a region of about 3000 sq. miles. The high mountains 

 (6000 feet minimum height) are chiefly made up of granitic rocks, with 

 no gneiss. On this mass in the Tiliri district are a few hundred feet of 

 Tertiary rocks dipping seaward at a decreasing angle. These beds are 

 highly metamorphosed close to the granites ; they form an extension of 

 the Great W. Indian Miocene deposits. The conglomerates in this 

 series are entirely made up of clay-stone pebbles, the source of which is 

 unknown, and they contain no traces of granitic or other crystalline 

 debris. Porphyritic dykes cut through both the granitic and Miocene 

 rocks. Pico Blanco, supposed to be a volcano, is merely an extreme 

 type of an ordinary granite mountain. Gold occurs in the metamor- 

 phosed Miocene rocks, and always at some distance from the granites, 

 but not in paying quantity. A few patches of Post-Pliocene rocks line 

 the coast, which is low and swampy. On the beach very pure iron- 

 sand occurs in large deposits. G. A. L. 



. Costa Eica Geology. Amer. Journ. ser. 3, vol. ix. p. 320. 



Corrects height of Pico Elanco, which is 11,877 feet. The volcanic 

 peak Irazu is all but extinct. 



. Note on the Age of the Cretaceous of Yancouver Island and 



Oregon. Amer. Journ. ser. 3, vol. x. p. 308. 

 Claims to have first, with Prof. Whitney, put on record the age of 

 the Nanaimo Coal. [See Selwyn, p. 129.] 



