606 



SCIENCE. 



[N. S. Vol. III. No. 



igneous rocks, which sliould be considered with 

 their massive equivalents rather than with sec- 

 ondary schists of similar constitution. 



Mr. H. W. Turner described the Arcliean 

 Gneiss in the Sierra Nevada. The western part of 

 Nevada was thought by the geologists of the 40th 

 Parallel Survey to be an Archean area which, 

 during Paleozoic time was a land mass, since 

 there are no known Paleozoic sediments resting 

 on it. It was supposed that Archean rocks ex- 

 isted in the Sierra Nevada, although the Juras- 

 sic age of the hornblendic granites of that 

 range, as stated by Whitney, was accepted by 

 the 40th Parallel Survey geologists. 



An area of such rocks is believed to exist in 

 the central part of the range, and is well ex- 

 posed in the canyon of the north fork of the 

 Mokelumne Eiver and its branches. The rocks 

 are chiefly gneisses with which are associated a 

 granite which differs ft'om the Jurassic granite 

 of the range in containing much potash felds- 

 par, and no hornblende, or very little. This 

 granite is indistinguishable from some of the 

 Archean granite of the Fortieth Parallel survey 

 collections. The gneisses vary much in com- 

 position, some of them being made up chiefly 

 of plagioclase, monocliuic pyroxene and biotite; 

 another type is composed of plagioclase, horn- 

 blende and biotite; others carry quartz, and 

 correspond nearly to a quartz-mioa-diorite in 

 composition. Titanite, zircon, apatite and 

 pyrrhotite are among the accessory minerals. 

 Some of the titanites exhibit a pleoochrism, 

 like that found by Lacroix to be characteristic 

 of that mineral in pyroxene-gneisses. Certain 

 light colored bands containing garnet, quartz 

 and a mineral resembling wollastonite, may 

 represent original limestone lenses, or may be 

 regarded as vein deposits. One stratum, sup- 

 posed to be a quartzite in the field, contains 

 much pyroxene between interlocking quartz 

 grains, and also numerous zircons. By far the 

 greater part of the area is made up of the 

 plagioclase-hornblende-biotite gneiss. The con- 

 tact of the series with the large mass of horn- 

 blendic granite lying to the east is sharp. 

 Apophyses of the hornblendic (Jurassic ?) gran- 

 ite extend into the gneiss and older granite as 

 dikes, and there are clear cut inclusions of the 

 gneiss in the late granite. All of the rocks 



composing this Archean complex are thoroughly 

 crystalline, and there is at present no positive 

 evidence that any portion of the mass repre- 

 sents original sediments. The area has a max- 

 imum diameter of about nine. miles. On the 

 west it is in contact with the great area of 

 Paleozoic sediments of the Gold Belt of the 

 Sierra Nevada. Its relation to this Paleozoic 

 series has not been made out. 



NATIONAL GBOGEAPHIC SOCIETY. 



At the regular technical meeting of this So- 

 ciety held in Washington, D. C, March 20, Mr. 

 Gilbert Thompson explained and advocated the 

 use of geodetic control lines in geographic work 

 as supplementary to primary triangulation, 

 when such lines are measured with care and 

 latitude and longitude determinations made, 

 etc. Following him, Mr. N. H. Darton read a 

 paper on the 'Physiographic Development of the 

 District of Columbia Region.' He outlined the 

 geologic history of the river from early Creta- 

 ceous time, mainly in its bearing on the cycles 

 of development. The present configuration is 

 the product of sculpturing and deposition in 

 Pleistocene times, but buried beneath the various 

 deposits there is a succession of older land sur- 

 faces. The earliest recognizable surface is the 

 floor of crystalline rock on which the Potomac 

 formation was deposited. This is exposed in 

 many points in the vicinity of Washington and 

 it is seen to be a relatively smooth peneplain 

 surface, which originally sloped very gently to 

 the east and southeast. Other similar plane 

 surfaces were eroded in the uplifts separating 

 the several later Cretaceous, Eocene, and Neo- 

 cene formations. These were widely extended 

 base levelings, which were part of the general 

 Tertiary planing of the Piedmont region. The 

 present topography began with the uplift of the 

 Lafayette, which amounted to about 120 feet. 

 As the land rose the Potomac river was born, 

 with its seaward course deflected by shoals on 

 the Lafayette surface. The minor drainage was 

 developed with approximately its present out- 

 lines, out more or less deeply. Then with slight 

 submergence with deposition, in which the early 

 Columbia formation was spread over the floor 

 of the wide river trough, and up the lateral 

 valleys. 



