54 Washington — Study of the Glaucophane Schists. 



General Conclusions. 



It will be of interest to compare the foregoing analyses with 

 each other, and for this purpose those which seem to be of 

 most value are collected in the large table on page 55. It is 

 at once evident that these rocks are divided into at least 

 two main groups. One, the larger, is basic, with Si0 2 ranging 

 only from 46 to 49*7, the other is very acid, with Si0 2 ranging 

 from 74-5 to 82*5. These two seem to be very sharply sepa- 

 rated, a possible third intermediate group being represented 

 by only one analysis, with Si0 2 58*3. 



Basic Group. — Taking up the basic group first, it is seen 

 that they are of fairly uniform composition in most respects. 

 A1 2 3 in most of them is rather low, from 13 to 15, being higher 

 only in II (17-34), III (19*74) and Y (16-88) and YII (18-36). 

 Iron oxides are high in all, and FeO uniformly surpasses Fe 2 3 

 (molecularly), in two cases (I and X) very much so. MgO 

 varies from 4'1 to 8*2, and there seems to be no correlation 

 between it and any other oxide, though there are indications 

 of a parallel variation of it and FeO. 



CaO shows the most striking behavior, running from 4*4 to 

 5*8 in some, and then with a break from 11 to 13 in others. 

 It would be natural to connect this behavior with some corre- 

 sponding difference in mineral composition, but I have failed 

 to find any trace of such. One would suppose that the rocks 

 rich in CaO might be prone to carry garnets, but, while they are 

 abundant in Y and IX they are lacking in II, 1Y, YI and X. 

 On the other hand they are quite wanting in I, III and YIII, 

 in which CaO is low. 



Na 2 remains very steady, varying only from 1*1 to 3*7, 

 while K 2 is present, as a rule, only in traces, being above one 

 per cent only in I (2*7) and YIII (1*7). H 2 is, of course, 

 present in considerable, but varying, amount, and C0 2 like- 

 wise is irregular, being often quite absent, and again high. 

 Ti0 2 , when determined, is always high, a fact connected with 

 the frequent occurrence of rutile and tit'anite as accessory min- 

 erals. F 2 5 and MnO are present only in traces. 



Acid Group. — Inasmuch as in this group Si0 2 constitutes 75 

 to 80 per cent of the rock, the distribution of the other oxides 

 through the remaining 25 to 20 per cent allows of a compara- 

 tively small range in variation. It may be noted, however, 

 that FeO is constantly higher than Fe 2 3 , that MgO is rather 

 high for such acid rocks, that CaO decreases regularly as Si0 2 

 increases, and that Na 2 is constantly higher than K 2 0, which, 

 except in XY, is present only in traces. The very high Na 2 

 in XIY is undoubtedly to be attributed to the presence of 

 albite. 



