1 1 HENR Y S. WA SHING TON 



rocks. These dike rocks might then with propriety be called 

 lindoite, but in view of their more porphyritic character and for 

 other reasons they will be referred to as quartz-syenite-porphyry. 



In this connection must be described two rocks found cut- 

 ting the gabbro at Nahant. The one, which occurs at Little 

 Nahant, is fine-grained, scarcely porphyritic, mottled gray, pink 

 and green, and is evidently considerably altered. It is essenti- 

 ally a hornblende-biotite-quartz-syenite, and resembles in most 

 particulars the rocks just described. The feldspar is apparently 

 mostly orthoclase, and the hornblende is of a similar peculiar 

 green, not highly pleochroic, and much of it seems to be 

 secondary. The biotite, which is primary and which occurs in 

 smaller amount, is the most notable feature. It occurs as stout 

 crystals with ragged edges, often partially altered to the green 

 hornblende. Its color is greenish-brown, and the strong pleo- 

 chroism is very striking and unusual ; parallel to the cleavage 

 deep grass-green, perpendicular to this reddish yellow-brown, the 

 absorption in the former direction being much the stronger. 



The second rock, which is found at the road-metal quarry at 

 Nahant as narrow dikes and small " schlieren " in the gabbro, is 

 a compact, almost aphanitic, rather dark gray rock without 

 phenocrysts. In thin section it shows a multitude of small 

 isodiametric crystals of biotite having a peculiar light yellowish 

 color and usual absorption scheme. With these are very many 

 small colorless diopside anhedra, which in parts of the slide 

 become more numerous. These, together with many magnetite 

 grains and some small apatite needles, are embedded in a color- 

 less or slightly yellow mass, which between crossed nicols is 

 seen to be composed of tabular orthoclase crystals with fewer of 

 a twinned oligoclase, the mesostasis being in patches either 

 alkali-feldspar or quartz, the latter less abundant. In places the 

 feldspar mesostasis extinguishes simultaneously over large areas 

 and becomes poikilitic in character, and in these areas auto- 

 morphic feldspar crystals are wanting. The tabular feldspars 

 are in fact best developed and sharpest when surrounded by 

 interstitial quartz, when in feldspar they are much less well 



