GEOLOGICAL AND MINERALOGICAL NOTES: NO. 5. 11 



The microscopic structure of this rock, as shown by a 

 selection from the numerous thin sections which I have pre- 

 pared from different outcrops is as follows : 



(1). From Eastern Point ; midway between Bass Rock and Brace's 

 Cove : Orthoclase, quartz, chlorite, uralite, magnetite, numerous small 

 grains of titanite. "With a high power objective, under crossed nicols 

 the feldspar and quartz present the appearance of a mosaic. The feld- 

 spars are microperthite intergrowths of albite and orthoclase. 



(2). From the outer side of Salt Island : Micropegmatitic quartz 

 and feldspar grains, the feldspar grains being tabular Carlsbad twins 

 (always microperthite) , augite, green hornblende, somebiotite, magne- 

 tite, iron pyrite, and large sections of colorless garnets in the micro- 

 pegmatitic quartz and feldspar areas. With high power objectives, 

 even the smallest feldspar grains are seen to be microperthite. There 

 are, also, some micro-zircons as inclusions in the feldspars The entire 

 section shows that the rock has been subjected to great strain, for 

 much of the hornblende, and some of the feldspars are crushed and 

 broken. Decomposition in the hornblende has produced feathery- 

 formed glaucophane. 



(3). Near Brace's Cove, southeast : Quartz feldspars, hornblende, 

 chlorite, glaucophane, limonite. The quartz and feldspars are ar- 

 ranged as in the other slides. The orthoclase which is microperthite, 

 micropegmatically arranged, has inclusions of hornblende, limonite 

 and quartz grains. The evidence of great strain and crushing force, 

 sufficient to separate the quartz grains from the feldspars, is easily 

 detected. In many cases a rim of chlorite surrounds each grain, while 

 in some instances the limonite surrounds the quartz and feldspar grains, 

 giving the section the appearance of a clastic rock, usual in all of the 

 granulites. 



Many micro-sections of this rock from various outcrops 

 have been studied, and the results all point to the conclu- 

 sion that this extensive formation in the Cape Ann horn- 

 blende-granite area has a granitic structure, and has crys- 

 tallized from the magma in an aggregate of small grains, 

 partially metamorphosed by plastic deformation subsequent 

 to solidification, a secondary metamorphism having t;iken 

 place through great pressure and strain from causes yet to 

 be determined, but probably due to faulting as shown on 

 the coast line in this contact, and which gives the rock its 

 granulitic structure. 



