Johnson and Warren — Geology of Rhode Island. 23 



Chemical Composition and Quantitative Classification. — 

 Material for chemical analysis was obtained by breaking off a 

 large number of good-sized fragments from the parts of the ledge 

 where quarrying had exposed a comparatively fresh surface. 

 These fragments were broken into smaller pieces and of these 

 about 3 kilos were crushed in a chilled steel mortar, care being 

 taken to avoid pieces showing any signs of alteration. The 

 entire lot was then sampled so as to yield a product representa- 

 tive of the average composition of the rock. 



The presence of so much titanium* makes the analysis of this 

 rock of more than usual difficulty, and accordingly especial pains 

 were taken to insure the correctness of the various determina- 

 tions. The results of the analysis with the molecular ratios and 

 their apportionment to the various "normal" minerals are as 

 follows : — 



* The writer is indebted to Professor Henry Fay of the chemical department 

 of the Massachusetts Institute of Technology for the details of a highly satis- 

 factory method, both in point of simplicity and accuracy, for the determina- 

 tion of the titanium. The principles involved present nothing new, but the 

 exact details of the method, do not appear to be known to petrographers at 

 least, and for the benefit of those who may have occasion to analyze rocks rich 

 in TiO-2 the directions for the carrying out of this method are given herewith. 

 By observing these carefully, excellent results will be obtained. Where 

 extremely accurate determinations are desired, a third repetition of the sep- 

 aration may be made. In two made during the present investigation, only 

 one one-hundredth of one per cent of iron could be detected in the Ti0 2 

 precipitate. Aluminium and manganese are probably included to about the 

 same amount. 



Fuse 0'4 to 0"6 gms. of finely ground ore with 6 to 8 times its weight of 

 sodium carbonate. Extract the mass with hot water, and decant the solution 

 through a filter. Boil the residue with 25 cc. of sodium carbonate solution, 

 filter, and then wash the residue on the filter paper several times with dilute 

 sodium carbonate solution. Ignite the filter and residue in a platinum cru- 

 cible at a low temperature until the filter paper is burned. Fuse with 12 to 

 15 parts of dry acid potassium sulphate for one-half hour. The temperature 

 of the fusion should be so regulated that the mass is kept in the molten con- 

 dition, but fumes of sulphur trioxide should escape only when the lid of the 

 crucible is removed. Cool and remove the fusion from the crucfble by means 

 of a platinum wire. Suspend the fusion in 200 cc. of cold water to which 

 has been added 100 cc. of sulphurous acid and allow to stand in a cool place 

 until solution is complete. 



Filter if necessary. To the solution add 125 cc. of acetic acid (sp.gr. l - 04) 

 and dilute to 800 cc. in a liter beaker. Add 20 gms. of sodium acetate dis- 

 solved in a small amount of water and boil from 3 to 5 minutes, adding just 

 before the boiling point is reached an additional 25 cc. of sulphurous acid. 

 Allow to stand in a warm place for one-half hour and then filter by means 

 of a siphon through a 9 cm paper. 



Wash the precipitate with 5 per cent acetic acid solution until most of the 

 sulphate has been removed and then ignite the paper and precipitate and 

 fuse with acid potassium sulphate again. Proceed exactly as before, wash 

 thoroughly, finally igniting and weighing the precipitate as TiO- 2 . The latter 

 in the writer's experience always has a light to dark gray color. 



