HORN EXPEDITION — GENERAL GEOLOGY. 43 



Specimen No. 97 is an e.xaniple of a gneiss which exhibits a nioi-e advanced 

 stage in the production of "eyes" tlian tliat just described. This variety of 

 gnei.s.s, which was met with on tlie north side of Brinkley BhilT, was observed to 

 extend over a larfje area in tiie McDonnell Ranijes. 



In this specimen the felspar and quartz are not so distinctly marked oflf into 

 folia separated by tiiin more or less parallel layers of biotite. They seem, however, 

 to be much crushed, and, together with the biotite, to form, as it were, a ground 

 mass to the lenticular portions. 



Thes(! leiis-shaped masses usually show when e.xamined carefully their relation 

 to the remainder of the rock, by the presence of narrow laminal running out from 

 them into the mass of the rock. 



Anothiu- specimen (No. 4) seems to mark a further stage in the process of 

 metamorphism ; it was obtained from an outcrop in the bed of Maude Creek, 

 which takes its rise on the southern slope of Hart Range, and is a typical 

 representative of a gneiss found extending over a lai-ge area. 



The finely-foliated structure is not so well marked in it as in the above- 

 mentioned specimens. There seems to have been a greater movement of the 

 constituents during crushing, the tine portions of which wrap round the lenticular 

 masses in such a manner as to simulate the " tlow-structure " so well developed in 

 many volcanic rocks with porphyritic crystals. 



The " eyes " are entirely isolated, and consist chiefly of (juartz and felspar, 

 while in the ground mass are to be seen quartz, felspar, biotite, and garnets. 



A gneiss from Belt Range (No. 3i) resembles in structure specimen No. i. 

 In the former, however, the "eyes " are composed of microcline, and are embi'dded 

 in a crushed mass of felspar, quartz, and a small quantity of decomposed mica. 



3. Those that are clearly of eruptive origin : — 



(a) Rocks of the Acid Group. 



"Uolgarna" granite. The granite from which the iiinnense crystals of mica 

 are oljtained, at the Oolgarna Claim in the Hart Range, is composed of the usual 

 constituents of teinary granite, viz., (juartz, f(ds[)ai', and mica. As the fi^lspar 

 belongs to the triclinic division, the rock ougiit to be termed a granitite. Although 

 all the constituents are developed on an enormously large scale, the crystals being 

 often from two to three feet in diameter, no one particulai' mincn-al is porphy- 

 ritically developed. The felspar is of two kinds — a potash variety, which invari- 



