﻿408 FERGUSON. 



Rare grains of epidote occur, generally along the darker bands. 



The groundmass in great part is composed of small rods and grains of a 

 practically isotropic mineral with a slight greenish tinge, which, with crossed 

 nicols, gives a very faint, grayish polarization or remains entirely dark. The 

 index of refraction is lower than balsam, hence they are either crystallites or 

 else small rods of opal. They have a wavy, parallel arrangement suggesting 

 flow structure (microfelsitie). Between these rods are aggregates of a mireral 

 having low polarization colors and suggesting chlorite. These sometimes have 

 definite boundaries suggesting the replacement of some other mineral, or possibly 

 only a large interspace between the small rods, or else they fill up irregular inter- 

 stitial spaces. The polarization is extremely irregular and parts of these aggre- 

 gates seem isotropic, hence they may represent a cryptocrystalline structure 

 resulting from devitrification. 



Small aggregates of a mineral having a high double refraction and resembling 

 either sericite or talc also occur, but these are rare. The banding seems to be 

 due to small patches of nearly opaque brownish matter. 



Below is an analysis made by Mr. Herbert S. Walker of this Bureau. 



Analysis of Cebu No. 57. 



Per cent. 



Si0 2 67.25 



A1 2 3 13.12 



Fe 2 3 -24 



CaO 1.23 



MgO 1.10 



K 2 4.38 



Na,0 0.59 



at 11° 6.15 



H..0 



on ignition 0.1 1 



Total 100.17 



The most striking features of the analysis are: (1) The rather large 

 ratio of aluminia to silica, 1 : 5.1, which is above what would be expected 

 in a siliceous precipitate; (2) the large percentage of both contained 

 and included water, which would be natural to expect in a chemical pre- 

 cipitate of the nature of a novaculite, having an opaline groundmass, but 

 which, if the rock is igneous, must be referred to devitrification and 

 chloritization of the groundmass, a process which seems hardly far enough 

 advanced to justify such a large amount of water ; ( 3 ) the extremely low 

 iron content, natural to a precipitate but hardly to be looked for in an 

 igneous rock as fresh as this appears to be; (4) the comparatively high 

 percentage of magnesia; (5) the great excels of potash over soda (7.4: 1 

 taking percentage values, or 4.7 : 1 taking molecular proportions). 



In addition to the high water content and the low percentage of iron, 

 there are two other arguments against the rock being of igneous origin. 

 First, the specific gravity (2.18) is abnormally low for an igneous rock, 

 but neglecting all the water this figure would increase to about 2.6, and 



