\V. II. HOBBS — SECONDARY BANDING IN GNEISS. 463 



The gneiss is mainly composed of quart/., feldspar and mica, and effervesces 

 slightly with acid. Thin sections show that it contains rather more muscovite than 

 biotite, and, as accessory minerals, zircon and magnetite. The dip of the dolomite 

 in the southern part of the quarry is 60°-70° west, and the strike about north 15° 

 west. Corresponding dips occur in the northern quarry. In ascending the hill the 

 gneiss is first met with at the spring (about 50 feet from the upper edge of the 

 quarry). Besides a cleavage-foliation (60°-70° east), the gneiss shows a marked 

 straight banding, the direction of which is the same as the foliation. No other struc- 

 ture than these two was discovered. A few paces above the spring an opening has 

 been made in the gneiss by a small quarry, and a similar opening has been made 

 about 100 feet farther northward. Here the mass of the rock is fresher and shows 

 the same structure as that at the spring, with the exception that much contorted 

 lenses of quartz clearly show the position of an earlier structure-plane, which now 

 has an average dip of 40° west, with steeper dips on the west and lower dips on the 

 cast. Locally other evidences of this structure can be made out, namely, a crumpled 

 banding having the same direction as the contorted quartz lenses. On the northern 

 wall of the more northerly of the two quarries this structure is brought out in great 

 perfection* The strike of the cleavage-foliation at this locality is north 15° west ; 

 the bedding-plane appears to strike about north 9° west. With the assistance of a 

 skilled stone-worker, a hloek of gneiss was cut from this spot so as to have it.- face 

 approximately perpendicular to the strike. The dimensions of the face of the hloek 

 were about two feet by one and its thickness about a foot. This block has been 

 sawed twice through parallel to its face and the plane surfaces of the slabs carefully 

 smoothed. One surface of the middle slab also has been given a polish. A photo- 

 graph of this surface is reproduced in plate 14. 



The unique feature of this specimen does not consist in the crumpled bands, the 

 contorted quartz lenses, or the cleavage foliation, all these having been observed 

 in other localities in Berkshire county ,f though it is doubtful if the three structures 

 have been observed together in such perfection as at this locality. The novel 

 feature is the secondary straight banding parallel to the induced foliation. This 

 banding is due to an alternation of layers of different mineral composition, which 

 gives the structure an appearance very like that of ordinary sedimentation. The 

 white bands are composed mainly of quartz and feldspar, the dark ones of mica. 

 These bands no doubt date from the same period and were produced by the action 

 of the same forces as the foliation. As already stated, the straighl banding and 

 foliation are the prevailing structures at the locality, the crumpled banding being 

 observed only al a few localities; and it is noticeable thai at these localities the 

 straighl banding dies out alto-ether as it meets the series of crumpled bands, to 

 recur again on the other side of them, as indicated in plate 1 1. 



The occurrence of parallel layers of different mineralogical composition in a meta- 

 morphosed elastic rock has been considered one of the besl criteria in determining 

 i he planes of stratification, where these have 1" 'en partially effaced by subsequently 

 induced structures. The structures observed in the gneiss of the Elopkins-Searles 

 quarry indicate thai one may easily be deceive 1 in applying this principle. 



Kxn.w \ i I0S OP Platj 11 



The plate shows u polished slab of ealcar is muscovite biotite gneiss from near the Hopkins- 



rli - dolomite quarry at Great Barrington, Massachusetts, ["he size of the bou.t 2 feet bj 



1 1 i- :ii-., well marked .i little way north of the quarry in a low led id can also be 



perfection, in blocks m found in the southern quarry, 



I lale, "i 



