PROCEEDINGS OF THE NATIONAL MUSEUM. 



669 



well, result from a recrystallization of the material derived by refusion 

 inaiuly from the older olivines and augites. The residual augites are not 

 separated from the groundmass by a sharp line, as might be imagined 

 from an examination of the figure alone, but pass into it by slight 

 gradations. 



The above-described minerals, together with small grains of iron 

 ore and innumerable minute, 

 greatly elongated, needle- 

 like crystals of a brownish 

 color, ;vnd which occur singly 

 or radiating in every direc- 

 tion from an indefinite nu 

 <?leus, complete the list of 

 recognizable constituents. 



A second variety, found in 

 inconspicuous outcrops a few 

 rods away, was described as 

 follows: 



Macroscopically this rock 

 consists of a compact apha- 

 nitic groundmass of dark 

 brown color, carrying abun- 

 dant greatly altered olivines 

 and augites. Under the mi- 

 croscope the groundmass 

 show^s a brownish, partially 

 devitrified base, traversed in 

 every direction by innumer- 

 able, short, thin, yellowish or brownish flecks of a dichroic mica-like 

 mineral, which are light yellowish when the plane of vibration of the 

 light is at right angles to the axis of greatest elongation and brownish 

 when it is parallel. Between crossed uicols these give maximum extinc- 

 tions and become almost completely obscured when their longer axes 

 coincide with the plane of either nicol, and are of a light yellowish 

 color at intermediate points. They are too minute and wath too imper- 

 fect outlines for a more accurate determination of their optical proper- 

 ties, but are undoubtedly of biotite or an allied mica. These flecks, 

 together with innumerable light greenish elongated augite microlites, 

 are so abundant as to form a dense, almost felt-like groundmass, in 

 which are embedded the abundant porphyritic augites and olivines. 



As above noted, both these minerals are badly altered though the 

 augite is still shown in a few sections in the form of broad, rounded 

 plates of a light greenish color, with sharply defined prismatic cleav- 

 ages, and containing very many large inclosures and embayments of 

 the groundmass. The olivine has completely decomposed, and but for 

 the characteristic crystal outlines of the pseudomorphs would be unrec- 

 ognizable. The product of this decomposition is in part a very light 



Fig. 10. 



CORRODED OLIVINES, AUGITES, AND SECONDARY MICA. 



. 38590, U S. N. M. 



