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(1). Charles Darwin (2) noted that unlike all other islands which rise from 

 oceanic ridges or from the abyssal plains, St. Paul's Rocks are not volcanic, but 

 rather consist of highly sheared (mylonitized) plutonic rocks. This has been re- 

 peatedly confirmed (1), and much additional interest has been stimulated by the 

 suggestion that these islet are in fact an exposure of the suboceanic mantle 

 (3,4). 



During cruise 20 of R.V. Atlantis II of the Woods Hole Oceanographic Insti- 

 tution, considerable work was done about St. Paul's Rocks, including the most 

 detailed geological survey to date (5). During this survey and from two large 

 dredge hauls taken from submarine talus slopes immediately south (dredge 7) 

 or southeast (dredge 18) of the islets during cruise 35 of R.V. Chain, we col- 

 lected mylonite samples of previously unrecorded compositional and mineralogic 

 heterogeneity (6). 



Descriptions follow based on the petrography of about 200 samples, on elec- 

 tron-beam microprobe analyses of mineral grains in many of these, and on the 

 complete wet-chemical analysis of four samples. Table 1 summarizes the latter 

 and gives an estimate of the average composition of the mylonites, based on the 

 observed relative abundances of mylonites of various types, both in near-shore 

 dredge hauls and exposed on the islets. Table 2 describes the mineralogy and 

 locality of each sample in Table 1. 



We have found it useful to classify the mylonites in two main mineralogic 

 types: spinel peridotite mylonites (after Tilley, 3, 7), and brown hornblende 

 mylonites, characterized by abundant brown, alkali-rich hornblende. There is 

 considerable variation among samples within each class. In addition, a third 

 mineralogically distinct mylonite was found rarely on Southeast Islet ; this type 

 consists mainly of colorless to light-green clinopyroxene, plagioclase, and scap- 

 olite, with subordinate brown hornblende. We suspect that additional petro- 

 graphic work will reveal other distinct rock types, but only as minority repre- 

 sentatives. 



Spinel peridotite mylonites, as described by Tilley (3, 7) and otheins (4, S) 

 are the most abundant rocks on all the islands, and in near-shore dredge samples. 

 Banding due to local concentration of amphibole is common on both Southwest 

 and Southeast islets ; amphibole may compose more than 50 percent of some 

 of these bands. Enstatite and chromian spinel are always present. Diopside is 

 commonly present when amphibole is scarce. Serpentine is restricted to joint 

 surfaces and composes less than 10 percent by volume of most of these mylonites. 



The second most abundant rock type, previously undescribed from the islets, 

 we have found to be brown hornblende mylonites, characterized by abundant 

 brown alkali-rich hornblende and a large and complex suite of associated miner- 

 als. These mylonites are minor constituents of both dredge hauls but occur in 

 situ only on Southeast Islet ; on this, the second largest islet, we estimate 20 

 to 30 percent of the surface consists of brown hornblende mylonite, interbanded 

 with spinel peridotite mylonites petrographically identical to those of the other 

 islets. The bands, a few millimeters (Fig. 1) to several meters thick, dip 60°E 

 and strike about NlOW, at right angles to the ridge from which the islets rise (1 ) . 



Particularly important features in unraveling the complex history of the 

 mylonites are large single or multimineral sheared-out grains referred to here 

 as augen (Fig. 2). The augen are thought to be relicts of the premylonitization 

 (primary) assemblages (3). Recrystallization and formation of new minerals, 

 particularly of hydrous or chlorine-rich phases, during and after mylonitization 

 have obscured the primary assemblages of many samples. For this reason, the 

 mineral assemblage of a mylonite matrix, although originally derived by granu- 

 lation of the augen, is not always the same assemblage as that of the augen. 



The spinel peridotite mylonites have been divided into two types by Tilley 

 (7) on the basis of the primary assemblages: mylonites with abundant amphi- 

 bole augen (the "amphibole type") and with abundant diopside and enstatite 

 augen (the "pyroxene type"). We have found that: (i) the "amphibole type" 

 is the most abundant type on all the islets; (ii) the amphibole augen are par- 

 gasite; (iii) augen of blue spinel (around SpsoHe2o), plagioclase, and phlogopite 

 are common accessories in the "amphibole type" ; (iv) augen of pargasite and 

 diopside may coexist in the same thin section ; and (v) the enstatite and diopside 

 of the "pyroxene type" are aluminous. 



The primary assemblages of the mylonites may have crystallized in markedly 

 different pressure-temperature environments. The olivine + aluminous en- 

 statite + aluminous diopside + chromian spinel assemblage of the "pyroxene 



