698 ARKHENIUS [CHAP. 25 



province of oceanic basalts, rather than transport from the circum-Pacific 

 andesite regions. MelHs (1959) has found examples of uralitization of pyroxene. 



The conditions determining the rate of decomposition and devitrification of 

 volcanic glass in marine sediments are still obscure. While some minute glass 

 shards in Mesozoic sediments are unaltered, some Quaternary deposits of ash 

 and pumice have been entirely altered to montmorillonoid minerals or to phil- 

 lipsite. The occurrence of unaltered glass in calcareous deposits indicates that 

 a high calcium ion concentration might preserve the glass. Layers and laminae 

 of volcanic glass are potentially useful as stratigraphic markers because of the 

 wide dispersal and the short duration of the supply of volcanic ash. Successful 

 attempts to correlate strata over large distances in marine sediments have been 

 made by Bramlette and Bradley (1942), Mellis (1954) and Nayudu (1962a). 

 A correlation has also been suggested between ash layers observed (Worzel, 

 1959) off the Middle American coast in the Pacific and in the Gulf of Mexico 

 (Ewing et al., 1959). Further observations of these deposits suggest, however, 

 that they are due to eruptions from different centers and probably at diff"erent 

 times. 



Kaolinite, which commonly occurs as a weathering product of basaltic 

 glass under acid weathering conditions (Sigvaldason, 1959), has not been found 

 in marine decomposition products of pyroclastics. This agrees with the experi- 

 mental observations of the influence of hydrogen ion concentration on the 

 stability of low temperature silicates, referred to below. 



Palagonitization of basaltic glass is frequently observed on the ocean floor; 

 Correns (1939) and Nayudu (1962) indicate that this alteration takes place 

 mainly at the interaction between hot lava and sea-water or interstitial water, 

 rather than during the continued exposure after cooling. 



The zeolite phillipsite was early recognized as quantitatively important in 

 slowly accumulating pelagic sediments (Murray and Renard, 1884). Recently, it 

 has been found that other members of the phiUipsite-harmotome series are 

 widespread, although less conspicuous because of their microcrystalline habit, 

 the low intensity of their first order diffraction and their diffractive interference 

 with feldspar. Zeolite concentrations higher than 50% are not infrequent, and 

 zeolitite consequently is a common pelagic sediment type. Controlled synthesis 

 of the mineral indicates phillipsite and harmotome to be stable from a lower pH 

 limit between 7 and 8 to an upper between 9 and 10 in a potassium-hydrogen 

 cation system. Above this limit feldspar is stable at low temperatures, and 

 directly below the limit IM mica forms from solution. These relations might be 

 useful in estimating the hydrogen ion concentration in ancient marine environ- 

 ments. 



It is not entirely clear how much of the silicon and aluminum of the zeolite 

 framework, and the interstitial cations, is directly inherited from the igneous 

 silicates and glass decomposing on the ocean floor, and what proportion is de- 

 rived from ions which have long been in solution and circulation in the ocean. 

 The amount of aluminum passing through the ocean in a dissolved state is of the 

 same order of magnitude as the rate of accumulation of zeolites (Sackett and 



