210 The Anierica/i Geologist. April, issis 
(lOo). The dichroisin mentioned by Hobbs* as character- 
istic of the augite of the diabase could not be detected in these 
bands. The chief alteration product on the cleavage cracks 
is chlorite; in several places a deep green amphibole was also 
observed, in well marked basal sections, and without fibrous 
structure. It seems probable that the formation of this 
mineral was associated with the same process that produced 
the inner zones, next to be described. 
THE ZONE OF POTASH FELDSPARS. (F.) Next 
to the augite wreath, on the inner side, is a row of squarish 
crystals of microcline, developed base to base with the prisms 
of the augite zone, in bunches idiomorphically terminated on 
the proximal side. (See figure i.) In two cases the character- 
istic microcline twinning is very plainly shown, though as the 
section is cut transverse to the surface on which the crystals 
developed, the basal section, which would show plainly the 
grating structure, is rare. The M sections are common, and 
in one case the extinction is 5°, with no twinning visible. The 
section nearest to the basal gives an extinction of 12° on one 
lamella of the albite twinning and about 17' on the other; the 
pericline lamellae are indistinct and show that the section is 
obliquely oriented, but the grating-structure is typical. 
Furthermore, these feldspars are very largely kaolinized, the 
opaque portions showing in reflected light the characteristic 
salmon color. 
THE ZONE OF MICROPEGMATYTE. (M.) Within 
this band of microcline is a zone of varying width consisting 
of quartz individuals filled with a very remarkable micropeg- 
matyte intergrowth with the kaolinized feldspar; the latter 
forms feather and fern-like structures of most delicate and in- 
tricate patterns, sometimes anastomosing, sometimes enclos- 
ing minute triangular portions of the quartz, and in other cases 
with the usual microgranitic habit. A portion of this struc- 
ture is shown in the accompanying microphotograph (figure 
2) magnified 60 diameters, in ordinary light; the light portions 
are quartz, the dark are kaolinized feldspar. 
In association with this, and nearer the quartz border, cal- 
cite and chlorite are abundant, the latter sending long fibrous 
bundles into the crevices of the quartz (O), the border of 
*Op. cit., p. 6. 
