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The National Geographic Magazine 



and increases until in a few of the speci- 

 mens it characterizes the rock as horn- 

 blende andesite. The hornblende is 

 always of the deep brown, strongly 

 pleochroic variety, with rounded black 

 border, which indicates that the horn- 

 blende during the later portion of the 

 molten stage of the lava was undergoing 

 resorption. Next to magnetite horn- 

 blende is among the earliest products of 

 crystallization in the magma, as it. rises 

 by eruptions from deep in the earth to- 

 ward the earth's surface. Arriving near 

 or at the surface, where the conditions of 

 pressure and temperature have greatly 

 changed, hornblende is no longer stable, 

 and is gradually attacked and resorbed 

 by the magma, to crystallize out upon 

 final solidification as pyroxene either as 

 augite or hypersthene. While it may 

 not be asserted that all the hypersthene 

 andesites of the collection once con- 

 tained hornblende, it is certain that some 

 of them did, and that if the molten con- 

 dition had continued long enough after 

 reaching the surface all the hornblende 

 would have disappeared. 



Arranging the andesites according 

 to their characterizing ferromagnesian 

 silicates, there are about a dozen hy- 

 persthene andesites, several of which 

 contain augite and a few hornblende. 

 There are two hornblende andesites car- 

 rying more or less hypersthene. The 

 hornblende andesite is associated with 

 hypersthene andesites, with and with- 

 out hornblende among the lavas of 

 earlier eruption. The product of the 

 late dsetructive outburst is hypersthene 

 andesite. 



EARLIER VOLCANIC ROCKS 



To illustrate the products of earlier 

 volcanic activity on Martinique previous 

 to the eruption of May 8, 1902, ex- 

 amples were collected of building stones 

 in St Pierre (Nos. 6, 7, 8, 11, and 13), 

 building stones in and near Fort de 

 France (Nos. 9, 12, and 15), and from 



Font St Denis (Nos. 10 and 14). The 

 other specimens embrace boulders from 

 Precheur (No. 1), fragments from Fu- 

 marole Island, mouth of Riviere Blanche 

 (No. 2) ; CarbetPeak ( Nos. 3a and 3^), 

 Pitons, Mount Carbet (Nos. 4 and 20), 

 and St Pierre (No. 5) , and black sand of 

 Beach Precheur (No. 31). 



Hypersthene Andesite. — Specimens 

 (No. 1) bearing the label "Precheur 

 boulders, old rock," are good examples 

 of hypersthene andesite. To the naked 

 ej^e the rock looks uniformly gray, and 

 only on a second closer look one per- 

 ceives that it is peppered full of small 

 crystals. Many are black, but most are 

 white. 



Under the microscope in thin section 

 these crystals stand out more conspicu- 

 ously in a gray groundmass, and the 

 structure is microporphyritic. The 

 white crystals are plagioclase, the dark 

 ones are mostly hypersthene, with some 

 augite and numerous grains of magne- 

 tite. The plagioclase in transmitted 

 light generally becomes clear and color- 

 less, and between curved nicols shows 

 lamellar twinning. Some are full of in- 

 clusions arranged more or less clearly 

 in concentric shells of crystal growth. 

 The angles of extinction of the plagio- 

 clase suggest that it is a lime-soda feld- 

 spar approximating the composition of 

 labradorite. 



The hypersthene occurs in small 8- 

 sided prisms, in which the four pre- 

 dominating planes are pinacoids. It is 

 strongly pleochroic yellowish to green- 

 ish, and has parallel extinction, distin- 

 guishing it from the greenish augite, 

 which is not pleochroic and has a large 

 angle of extinction. 



The groundmass in which all these 

 crystals are imbedded is dark gra3\ and 

 contains a multitude of embryonic crys- 

 tals, chiefly, if not wholly, of the min- 

 erals already mentioned, swimming in a 

 clear glassy base. 



From "beach at Precheur" is an in- 

 teresting specimen (No. 31) of "black 



