ANALCITE CROUP OF IGNEOUS ROCKS 68 1 



one would expect all the more to find the acid complementary 

 fornis — the oxyphyres — which accompany them also in, at least 

 partly, glassy forms. This, however, is not the case in any of 

 the regions, so far as we know, in which these rocks have been 

 found, where erosion has cut down so as to expose the deeper- 

 seated intrusive types. 



The question thus raised on geological grounds concerning 

 the glassy nature of the base was sought to be settled by other 

 means. Since the microscope failed to yield any decisive results, 

 recourse was had to chemical methods. In a number of samples 

 that have been analyzed were some free from biotite and segirite, 

 and in which consequently all of the alkalis and presumably the 

 alumina and water were in the base. In such cases it is noticed 

 that the molecular ratios of these elements have approximately 

 the relation : 



AI2O3 : K.^O + Na,0 : H,0 :: i : i : 2. 



If sufficient silica is deducted to satisfy the lime, iron and 

 magnesia as pyroxene and olivine, the remainder has an approx- 

 imately molecular relation to the above as follows : 



S : O,, : Al,03 ■ Na-.O -f K^O + H,0 :: 4 : i : i : 2. 



These, however, are the ratios given by the chemical for- 

 mula of analcite. The results, of course, cannot be very accu- 

 rate, as the exact composition of the crystallized minerals is not 

 known, but are sufficiently close to show that grave doubts must 

 exist concerning the glassy character of the base. Evidently to 

 settle the question the base would have to be separated and 

 analyzed. This has fortunately already been done by Hunter 

 and Rosenbusch in their original investigation, and, as the 

 results to be presently given will show, with great care and 

 skill, on excellent material. 



In separating the base from the included minerals by heavy 

 solutions it was found impossible to obtain it absolutely free 

 from microlites of ferro-magnesian minerals, and as Rosen- 

 busch states, a portion of these were attacked by the acid used 



