158 



THE AGRICULTURAL NEWS. 



M.w 7, 1904.. 



SOME GRENADA LAVAS. 



The following notes on 'Some Grenada Lavas' have 

 been communicated bv Profes.sor J. B. Harri.son, C.M.G., 

 M.A., F.I.C'., F.C.S.,' F.G.S., Government Analyst, 

 British Guiana : — • 



AVhen examining the rocks collected in Grenada m 

 connexion with the soil examinations made by me in 

 1895-G, the re.sultsof which were published in 1897 under the 

 title of 'The Rocks and Soils of Grenada,' the microscopical 

 examinations were made in London with slices, which I had 

 jirepared in Kritisli Guiana, and whidi frum lack of practice 

 I had not succeeded in making as thin as desirable. The 

 matrix of rocks of two types, which were classed as Augite- 

 andesite with olivine and a.s olivine-ba.'^alt, was described by 

 the gentleman who examined them for me as feldspathic. 

 At the time I recognized that the analy.ses indicated that 

 the rocks were of a more basic nature than either andesites 

 or feldspathic basalts should be, but I was so fully occupied 

 witli other work that I did not investigate the cause of this. 



During a hurried visit to St. C.eorge, Grenada, in 1902, 

 Tfhile in tran.sit to Trinidad, I collected some further 

 .specimens of lava from some blocks lying in beds of volcanic 

 conglomerate near the mouth of the Sendall tunnel on the 

 shore of St. George's bay, and I .sent these together with 

 some of those which, in company with Mr. Broadway, I had 

 collected in 1895, to Messrs. Voigt and Hochgesang, and had 

 thin sections j>repared for microscopical examination; these 

 were made sufficiently thin to allow a satisfactory examina- 

 tion of the matrix being made. 



The lavas are made up of phenocrysts of a green augite 

 and some of plagioclase-feldsjiar with some small blebs of 

 olivine in a felspathoid matrix of granules and prisms of 

 iiepheline, granules of colourles.s and of green augite ; grains 

 cf olivine and some grains of original and of secondary 

 magnetite, the latter being derived from the green augite and 

 from olivine. The lavas are therefore according to tlieir 

 matrix augite-nephelinite and nepheline-basalt, the matrix of 

 the latter being very rich in green augite and in olivine. 



The analj'ses given in 'The Rocks and Soils of firenada" 

 are as follows : — 



The augite-nephelinite is a rock of the albanase type, 

 while the nepheline-basalt is allied to lielchrose. 



The ultimate magnatic compositions of the lava.s are as 

 follow.- : - " 



At the time the original examinations were made the 

 green augite was separated from the matrix and found to 

 have the following composition : — 



If we take into consideration the comiposition of the 

 green augite which is present in both da.sses of tlie rocks, 

 their niineralogical compositions are probably as follows : — 



The silica shown in both of the above c;dcuIations is 

 doubtless secon<lary as many of the specimens collected 

 showed signs of commencing deconiiwsition. When small 

 pieces of the rocks are placed in cold diluted hydrochloric 

 acid of specific gravity LOo, gelatinous silica is set free, 

 the rocks are decomi>iised, and in the course of a few hours 

 fall to pieces, a .sandy residue being left, the dark coloured 

 augite resisting the action of the acid while the fel-jiathoid.s 

 and tlie filivine are <iuickly attacked and decomposed by it. 



The foregoing notes will to many a])pear out of place in 

 a journal devoted to agriculture, but this is not so. They 

 exi>]ain the formation, rapid production, and texture of the 

 soils of great fertility in s(;veral parts of Grenada through 

 the easy degradation, due to their niineralogical rc^inposition,. 

 of certain of its lavas to great depths. 



If similar rocks nejAelinites or iiephelin-biisalts — occur 

 in other i>arts of the West Indian Islands, soils of likt- i>rc«pertie.- 

 to the Grenada ones will result from them. 



