199 



TABLE II 



DISTRIBUTION OF ROCK TYPES IN DREDGES 2 AND 3 



Number of fragments which weigh more than 5g 

 dredge 2 dredge 3 



1 Some of these contain small amounts of deuteric chlorite and, rarely, actinolite, and are more 

 appropriately termed diabases. 



from the eastern slope of the median valley. Other dredges in the area revealed 

 unmetamorphosed basalts which are increasingly "weathered" west of the median 

 valley. Table II gives the distribution of rock types in the dredges. The fresh igneous 

 rocks from the 22°N area are presently being studied, but the preliminary results of 

 that study are not included in this paper. 



The area of the Mid-Atlantic Ridge from which the dredge samples were 

 obtained was surveyed in detail during cruise 44 of R. V. "Chain" (Van Andel 

 et al., 1965) and cruise 1 of R. V. "Thomas Washington" of the Scripps Institution of 

 Oceanography in 1965. The bathymetric survey was controlled by means of celestial 

 fixes and a set of radar reflector buoys. A generalized bathymetric map is shown in 

 Fig. la. In this area the ridge possesses a well-developed median valley with a maxi- 

 mum depth of 4,200 m, bordered by two crestal ridges with minimum depths between 

 1,200 and 2,400 m. The flanks of the valley are generally steep, ranging from 15° to 

 45°. The dredge hauls are from the northern part of a long, slightly curved, steep 

 slope probably representing a major fault scarp. The slope of the section dredged 

 averages 33° and consists of three straight slope sections separated by short steps. 

 Such three step slopes are common along both sides of the valley in this area. 



Analytical methods 



The greenstones were analyzed by traditional wet chemical means by E. Jarosewich 

 of the Division of Meteorites. Mineral compositions were determined with an A.R.L. 

 electron microprobe by W. G. Melson. Homogeneous minerals which are similar 

 in composition to those in the greenstones were used as microprobe standards, and no 

 corrections were made for the slight diff'erences in matrices. Determinations of chlorite 

 compositions are based on a chlorite standard analyzed by D. Foster of the U. S. 

 Geological Survey (unpublished, U.S.N.M. 109481). Actinolite compositions arc 

 based on an actinolite kindly furnished by Dr. F. R. Boyd Jr., and a hornbli nd(. 

 furnished by Dr. Brian Mason. Both were analyzed by H. B. Wiik. Plagioclasc cc^m- 

 positions were determined using glasses of plagioclase comipositions. Cplical r.rd 

 X-ray diffraction data are also given for some minerals. 



