CONSTITUTION AND ORIGIN OF SPHERULITES. 415 
sees in many of them the even more mysterious substance 
called microfelsite, which appears to be abundant in the rocks 
studied by certain petrographers, is never mentioned by other 
good observers, and has certainly never been found by any 
one in such development that its right to recognition as a 
definite substance could be established. 
“ Mikrofelsit ” is a term proposed by Zirkel for the micro¬ 
scopically felsitic, i. e., unresolved and apparently unindi¬ 
vidualized, yet not glassy, base of acid porphyries and rhyo¬ 
lites. Whether desirable at the time of its proposal, or not, 
the term soon fell into disrepute, because used as a mere 
“ confession of ignorance ” by careless observers. The latest 
definition of microfelsite, and that with which we are here 
concerned, is given by Rosenbusch in his already cited work. 
The definition is scattered through the descriptions of several 
rocks, but when the parts are put together it is seen that 
Rosenbusch gives to microfelsite the properties of a distinct 
mineral species. It is averred to have a stoichiometric 
chemical composition and a homogeneous crystalline struc¬ 
ture. It occurs in scales and fibers which normally do not 
have any effect upon polarized light, and the radiate spher- 
ulitic form is characteristically assumed. Through strain of 
some kind such spherulites give a negative black cross 
between crossed nicols, and even superposed scales in a 
groundmass may acquire the power of polarizing light. 
The chemical composition assigned to microfelsite by 
Rosenbusch is that of a silicate of alumina and the alkalies. 
The bases have the ratio 1:1, as in feldspar, but there is 
more silica than in any feldspar. The ratio of silica to bases 
is not given, and is apparently not constant. How can the sub¬ 
stance, then, be asserted to have a stoichiometric composition? 
There are obviously many reasons for doubting the cor¬ 
rectness of the position taken by Rosenbusch in regard to 
microfelsite. Teall * and Brogger,f among others, have defi- 
* Teall (J. J. Harris). British Petrography. 4 °. London, 1888 , p. 402 . 
f BrOgger (W. 0 .). Die Mineralien der Syenitpegmatitgange, etc., being 
Zeitschrift fur Krystallographie und Mineralogie etc. von P. Groth. 8°. 
Leipzig, 1890 , vol. 16 , p. 402 . 
