2g2 



The National Geographic Magazine 



dish brown. Under the microscope it is 

 seen to be made up very largely of clear 

 transparent particles, with others of 

 light cloud appearance. They are about 

 equally numerous. The clear ones gen- 

 erally show the lamellar twinning of 

 labradorite, and look like those of the 

 volcanic dust. The clouded material 

 when crushed is found to be an aggre- 

 gate of glass particles. Traces of hyper- 

 sthene and augite and magnetite occur, 

 but generally they appear to have been 

 removed by the action of acid gases in 

 the fumarole. Soluble salts may be 

 present, but could not be determined 

 microscopically. Mr Steiger tested one 

 of the specimens (No. 25), but was 

 unable to prove the presence of native 

 sulphur. The material throughout ap- 

 pears to be volcanic — sand and not de- 

 composed rock nor to any great extent 

 deposits from the escaping gases. 



Fragments from the Late Eruption on 

 St Vincent. — Mr Hill did not visit St 

 Vincent, but submitted a collection of 

 three small specimens, Nos. 17, 18, 

 and 19, of very vesicular lava from St 

 Vincent presented him by Lieutenant 

 Penny, of the Dixie. The exact loca- 

 tion and date of eruption are not given, 

 but it is presumed that they are pro- 

 ducts of the last eruption. 



Specimen [7 is light-gray pumice, 

 peppered with dark grains, but readily 

 floats on water. Under the microscope 

 it is seen to be made up chiefly of glass, 

 which is rendered 5'ellowish by dust- 

 like particles. The glass is very vesic- 

 ular and incloses crystals and fragments 

 of plagioclase, with less hypersthene 

 and a small amount of augite. Black 

 grains of magnetite are numerous. The 

 feldspar generally shows multiple twin- 

 ning bands, but a few squarish sections 

 are free from them. Many have well- 

 marked zones of growth and are full of 

 inclusions of dark glass with bubbles. 



The hypersthene is strongly pleo- 

 chroic from reddish yellow parallel to 

 the lateral axes to pale green parallel 



to the vertical axis, and with parallel 

 extinction. Cross-sections are nearly 

 square, with the corners cut off parallel 

 to the prismatic cleavage. There is but 

 a trace of macropinacoidal cleavage. ■ 



The pale-green augite is less common 

 and generally thicker, short prismatic 

 crystals, with large angle of extinction. 



A group of hypersthene and magne- 

 tite has for its center a lighter-colored 

 clear grain, whose high index of refrac- 

 tion and birefrangence indicates olivine. 



No. 18 is like 17, even to containing 

 olivine, and contains also a trace of 

 hornblende. In No. 19 neither olivine 

 nor hornblende were found, and this is 

 regarded as more likely to be the normal 

 rock of the eruption than the others. 

 An analysis by Mr George Steiger is 

 given (4) in the table, page 291. Such 

 associations of olivine is unusual, and 

 suggests early secretions. 



PROF. RUSSELL'S COLLECTION FROM 

 ST VINCENT 



Professor Russell kindly sent me three 

 specimens, one of lapilli and two of dust, 

 "that fell on May 7, 1902. These sam- 

 ples represent fresh material erupted 

 from L,a Souf riere. ' ' The lapilli had 

 the following label: "Average sample 

 of material which well at Georgetown, 

 St Vincent, on May 7, 1902. The ma- 

 terial of this nature which fell ranges 

 in size from fine dust up to rough frag- 

 ments 5 and 6 inches in diameter. Min- 

 gled with this fresh lava are angular 

 fragments of fine-grained, bluish com- 

 pact rock, representing older terranes, 

 which came down red hot and broke 

 into small pieces on striking. Collected 

 May 27, 1902." 



No. 1. This specimen is about five 

 centimeters in diameter, reddish brown, 

 and spotted white and black, with crys- 

 tals of feldspar and pyroxene. It is 

 pumiceous, but not sufficiently light to 

 float on water. Under the microscope 

 it is mainly vesicular glass, containing 



