246 G. C. Martin — Dunite in Western Massachusetts. 



gives a uniform interference color. These groups are then the 

 remains of former olivine crystals which are nearly uniform 

 in size (*3 to l mm ) and interlock to fill the entire field of the 

 microscope. The cracks between the original olivines are no 

 better developed than those between the shattered fragments. 

 The serpentine is generally in fine cracks but often has recrys- 

 tallized into needles varying in size from *02 by "1 to "04 by 

 •5 mm . These in general follow the olivine cracks. But often 

 there is a parallel arrangement of the serpentines and some- 

 times a tendency for all the serpentines of an individual olivine 

 to arrange themselves in two directions nearly at right angles, 

 — thus giving an appearance resembling Hussak's "gestrickte 

 Structur," but showing by the enclosed olivine its derivation 

 from that mineral instead of from pyroxene, as is usually 

 assumed for that form of serpentine. 



A specimen of this rock was powdered and the magnetic 

 ingredients removed. Then by means of the Thoulet solution 

 the serpentine was floated off. There then remained olivine 

 with black grains scattered through it. The latter came down 

 slightly in advance of the olivine and by repeated separations 

 pure samples of each were obtained. The black non-magnetic 

 grains had a brown streak and gave reactions showing the 

 presence of iron, chromium, and aluminum. They are then 

 doubtless at least partly picotite. The magnetic grains showed 

 a trace of chromium. An examination of the olivine showed 



Color pale green 



Sp. Gr. 3-273 (not corrected for temperature, etc.) 



Composition _ 



MgO .'_ 51-41 



Si0 2 40-07 



FeO 4-84 



A1 3 3 1-94 



CaO 



H 2 (ignition) 1*03 



99;29 



The remaining rocks of the area range in density from 2*9 

 to 2-54. As the density decreases the macroscopic characters 

 of the rock above described are gradually lost. The lightest 

 rocks vary much from each other in texture and in color. 

 Some are finely granular and homogeneous, some are schistose 

 or shaly, and some are fibrous; some are black, light or dark 

 green, yellow, white or mottled. But all grade, with the 

 density, toward the type described above. 



When these rocks are examined under the microscope it is 

 found that the amount of olivine decreases with the density. 

 And as the olivine decreases in amount, the serpentine increases 

 by a gradual widening of the cracks. Recrystallization of the 

 serpentine also takes place. Variation in rocks with the same 



