198 



EOCK DISmTEGEATTON" AND DECOMPOSITION 



Analyses of Trbsh and Decomposed Phonolite, Bohemia 



OONaTITtrBNTS 



Silica (S1O2) . . 



Alumina (AI2O3) . 

 Ferric oxide (PegOa) 



Lime (CaO) . . 



Magnesia (MgO) * 



Potash (K2O) . . 



Soda (NagO) . • 



Igmtion . . • • 



w 



O 



55.67% 

 20.64 



3.14 



140 



42 



5.56 



7.12 



4.33 



98.28% 



II 







Pi » 

 p p-i 



55.72% 

 22.19 



3.44 



1.28 



0.44 



6.26 



2.66 



7.79 



99.77% 





to 



to 



50 CO 



O O 



1^ o 



IV 



i ^ 



Eq O 



o o 



4.83 % 

 0,37 

 00 

 0.25 



0.001 



4.79 



0.001 



10.26 % 



91.46% 



98.40 

 100.00 



83.66 



95.65 

 100.00 



34.01 

 100.00 



^Bo 



8.64% 



1.60 



100.00 



16.34 



4.35 



0.00 



65.99 



0.00 



This phonolite, it should be remarked, consisted essentially 

 of sanidin feldspars and a soda zeolite, together with accessory 

 augite, black mica, magnetic and titanic iron, and possibly 

 hanyne. The zeolite is assumed to have originated from the al- 

 teration of the nepheline. The process of decomposition would 

 seem to consist, then, in the breaking down of this zeolite, and 

 the conversion of the rock into an earthy mass, with little other 

 change, so far as ultimate composition is concerned, than a loss 

 of a considerable proportion of its soda, and an assumption of 

 nearly 3.5% of water. The decomposed rock yielded 55.44% 

 of material insoluble in hydrochloric acid, with essentially 

 the composition of sanidin, showing that this mineral underwent 

 only a physical disintegration, the decomposition proper being 

 limited to the other constituents.^ 



Weathering of Diabase. — Turning to still more basic rocks, 

 we will next consider a disintegrated diabase occurring in the 

 form of a large dike extending from Granite Street in Somerville, 



^ Gain. The calculations for potash in column IV gives: 107.79% and for 

 ignition 164.77%. 



2 In calculating these analyses, it was found that the loss of alumina had 

 exceeded that of iron oxide, necessitating the assumption of the last-named 

 as a constant for comparison. The apparent gain in potash is presumably 

 due to errros in analysis, since,* as will be noted, the analysis of the fresh 

 material, given in column I, foots up only 98.28%. 



