THE BECKET CONGLOMERATE-GNEISS. 35 



A fiue-grained biotite-granitoid gneiss of gray oolor, with shade of 

 brown. The deep-brown biotite is scattered through a fresh colorless mix- 

 ture of quartz and feldspar. Titanite is so abundant as almost to deserve 

 place as an essential constituent. The lens shows a wholly even-grained, 

 very dusty mass. 



Under the microscope the quartz is characterized by the small number 

 of inclusions it contains, rarely fluid pores with large, slow-moving bvibbles 

 in the largest grains. The long rutile needles are wholly absent; stout, 

 flat muscovite microlites occur. Orthoclase appears in large, clear grains. 

 Microcline is the most abundant and the most recent feldspathic con- 

 stituent. 



Biotite in deep greenish-brown, jagged grains fits itself to all the other 

 constituents, and so is of later formation. 



Muscovite appears in small quantity under the microscope. 



Titanite appears in large, well-formed crystals, Avine-yellow, and in 

 abundant smaller, irregular-clustei-ed grains. 



Minute zircons, highly refractive, elongate, with rounded outlines, are 

 not rare. 



Magnetite and titanic iron are wholly absent. 



2. Granitoid gneiss from Clark Hill quarries, Middlefield. Coarse 

 quarry stone. 



A medium-grained, light-gray muscoAate-biotite-granitoid gneiss, whose 

 clearer color, as compared with the preceding, is produced by the increase 

 in the size of grain of the other constituents, while the mica does not increase 

 in size or quantity. The lens shows larger, limpid grains scattered in a 

 disconnected, granular, dusty, and micaceous groundmass, which is identical 

 with the whole mass of the preceding variety. 



The quartz rarely includes rutile needles, and contains, especially in 

 the larger grains, sheets of pores, often negative crystals, a few with large 

 motionless or slow-moving bubliles. 



The orthoclase is in subporphyi-itic masses, rendered turbid, as 

 usual, by an opaque white substance (kaolin?), which also occurs as an 

 exquisite dendritic growth thrust out among the fissures between the quartz 

 grains and appearing black by transmitted and sih-ery white by reflected 

 light. 



In one quadrangular section of orthoclase cut about parallel to oo P ob 



