494: Pirsson and Washington — Geology of JSfew Hampshire. 



Plagioclase in short thick tables and laths : mostly anorthite 

 but often zonal with varying mantles of labradorite ; Carlsbad 

 and albite twinning general; altering in spots, mainly in the 

 contact facies, into scapolite, which fills the interstices between 

 the other minerals. Feldspars embedded ophitically in horn- 

 blende, less commonly in biotite and augite. 



Biotite varies locally in amount ; in roughly developed 

 tables up to l mm ; ordinary brown variety and strongly pleo- 

 chroic ; slight variable openings of axial cross ; of uniform 

 coloration and not zonal ; intergrown with augite and horn- 

 blende and may include any of the other minerals ; altering in 

 places into chlorite and epidote, or into muscovite. 



Iron ore in anhedral to subhedral grains, sometimes includ- 

 ing cores of pyrite ; rarely altering into a deep reddish semi- 

 transparent substance assumed to be gothite; older than and 

 enclosed in the other minerals. 



Titanite in irregular grains, anhedral ; sometimes aureoling 

 iron ore ; sometimes in masses tilling angular interspaces 

 between automorphic feldspars ; sometimes independent in 

 form. 



Apatite not especially abundant ; in the usual prismoid crys- 

 tals enclosed in other minerals. 



Scapolite occurs locally distributed and appears to be chiefly 

 if not entirely confined to areas near the contact. It forms 

 irregular masses tilling the interspaces between other constitu- 

 ents, and its mode of occurrence and relations to the feldspars 

 show clearly that it is of secondary origin and formed at their 

 expense. From this it follows that it probably belongs to the 

 meionite end of the series. It is easily identified by its very 

 definite optical properties. 



Chemical Composition. — The chemical composition of this 

 rock is shown in No. I of the adjoining tables of analyses. 

 For reasons previously stated, it was made upon a specimen 

 not far from the border and representing rather more the bor- 

 der facies than the main type. As this contains more biotite, 

 as shown in the descriptions of the mode, the water is largely 

 constitutional, as hydroxyl in the biotite and hornblende, and 

 is not to be regarded as a necessary sign of alteration. 



The striking features of this analysis are the low silica and 

 alkalies with the high alumina and bivalent oxides. It is clear 

 that a magma of this character would form chiefly ferromag- 

 nesian minerals and labradorite. Especially notable is the large 

 amount of iron. 



For comparison the analyses of three other rocks (Nos. II, 

 III and VII) which are composed of similar minerals, and which 

 in the prevailing classification would be termed essexites, are 

 given. It has features of agreement with them and also differ- 



