100 abstracts: geology 



GEOLOGY. — Economic geology of Richmond, Virginia, and vicinity. 



N. H. Darton. Bulletin U. S. Geological Survey No. 483. Pp. 



48. with maps, sections, and views. 1910. 

 The Richmond region belongs in part to the Piedmont province, in 

 part to the Coastal Plain. Granite of undetermined age constitutes the 

 oldest rock of the area, and is succeeded by sedimentary formations as 

 follows: (1) Patuxent (Lower Cretaceous), Aquia, (Eocene), Calvert 

 (Miocene), Lafayette (Pliocene?), and terrace alluvium (Columbia). 

 All these formations except the Columbia dip gently to the east and are 

 separated by unconformities. A. H. Brooks. 



GEOLOGY. — Reconnaissance of the geology and mineral resources of 

 Prince William Sound, Alaska. U. S. Grant and D. F. Higgins. 

 Bulletin U. S. Geological Survey No. 443. Pp. 89, with maps, 

 sections, and views. 1911. 



Prince William Sound lies within highlands which form a part of the 

 Chugach Mountains. The fairly accordant tops of these mountains, 

 composed of highly folded strata, suggests an elevated peneplain which 

 has been warped and highly eroded. In this erosion glaciation has 

 played a prominent part. The topography of Prince William Sound is 

 that of a maturely eroded mountainous district with the forms of river 

 erosion modified by ice erosion. Into such a district the sea has come, 

 filling the main basin of the sound and extending far up the valleys that 

 lead into it. The evidence of glaciation are smoothed and striated rock 

 surfaces, roches moutonnees, hanging valleys, U-shaped valleys, fiords, 

 etc., with some deposits of till and glacial gravels. 



The sedimentary rocks of the region are chiefly closely folded slates 

 and graywackes, separable into two unconformable series, a lower, the 

 Valdez group, and an upper, the Orca group. Locally a conglomerate 

 formation occurs at the base of the Orca, with which is also associated 

 a large amount of greenstone. These greenstones are altered basic 

 lavas having in many places an ellipsoidal character. Intruded in these 

 series are granites and acid and basic dikes. 



The copper-bearing lodes occur along zones of fracture and mainly in 

 the greenstones, but some are in slates and graywackes, and some in the 

 contact between the sediments and the greenstones. Mineral deposi- 

 tion in the shear zones in part followed open spaces, in part impregnated 

 the country rock. The deposits are not definitely known to be related 

 to intrusives, but some basic dikes occur which may have had some miner- 

 alizing influence. A. H. Brooks. 



