PETROLOGY AT GOOSE CREEK — SHANNON. 65

Fig. 17.— Prehnite.

a side view of crystal a g g re-

gate similar to

that shown in FIGURE16.

barely perceptibly.

The main

portion of the crystal

shows

inter- ference colors mostly in

subnormal

blue

and

liver

brown

with a very little first-order yellow. All portions of the pattern yield a similar interference figure in convergent hght,

biaxial positive,

2V

approximately 10°,acute bisec- trix perpendicular to the table, axial plane parallel with the long direction,

r>v

strong. These inter- ference figures are

somewhat hazy and

confused.

This is

most

probably like the preceding, but the portions of scales

making up

the "hourglass" are so exceedingly thin that they do not greatly obscure the optical properties of the

main

crystal, even

though

the latter isitself very thin.

The

explanation implying

an

overgrowthof scales isnotpurelyhypothetical,_especially since

any

of the crystals

when

care- fully

examined

un- der a lens do

show

such scales, indeed usually a

group

of them, curving

upward.

_This is

similar to the tendency of the crys- tals ofprehniteto

form

sheaves

and

the flat crystals, with a thin overlyingscale, grade into bundles ofcurved, scalelike crystals.

Two

such aggregates are illustrated in figures 16

and

17; the

form shown

in figure 17 is very

much more common

than

any

other

and

has been _{referred to} as "dumb-bell" prehnite.

Drusy

surfaces are often

made up

of this type grading _{into still}

more

globular forms

and

the crystals of the preceding descriptions rest,in

most

cases,

upon

such shapes.

In

an

east-west shear zone exposed just south of the centerof thequarry in August, _1923, there were found

some

specimens ofprehnite

made up

of pale yellowish- green

columnar

_{bladed crusts}

up

to 1 cm. thick lining an

open

space.

The

surfaces ofthesecrusts are

smooth

botryoidalbutare

made up

ofthe terminationsof

innum-

erable closepacked crystals.

Attached

to thiscrust as thoughlater are single. well-definecT crystals ofprehnite

up

_{to 3}

mm.

broad,

which

are

more abundant and more

perfect

where

the space

between

thecrustsisnarrow. These

have

the crystalhabit

shown m

figure _11,

showing

the prism

m

(110), the front pinacoid 94110—24 5

Fig. 18.—Prehnite showing opticalstruc- ture OF CRYSTALS OF TTPE2.

c

mc:

Fig. 19.—prj-jj.

nite showing optical struc- ture ofcrys- tals of type3

66 PROCEEDINGS

OF

THE

NATIONAL,

MUSEUM.

VOL.66.

a (100),

and

the base c (001).

They

are thick tabular, parallel to the base,

and

areelongated

on

the haxis,as contrasted with the fore- going,

which

are elongated on the

d

axis.

The

pinacoid

a

(100) is

etched dull but the prismatic facesgive

good

reflections, indicating the angle

m

(110)

a m'"

(iTO)

=

^80° 39'.

The

basal pinacoid is striated parallel to the h axis as indicated in the drawing.

Between

crossed nicols these crystals also

show

optical anomalies, the patternbeingas

shown

infigure 18.

These

are in

many

respects like the ones previously described but they are

somewhat more com-

plicated.

When

the sectors c

and

c' are at the position of extinc- tion a, a'

and

^h

and

l' are similarly illuminated

and show

a uni-

form

first-orderyellow interference color.

These

sectors give

sym-

metrical extinction of 8°

on

either side of the dividing line.

The

extinction is not uniform, however, but sweeps as a bar

from

the innertip of the sector outward.

At

45° position all sectorsaresimilarly il-

luminated, a

and

a', h

and

b'shade

from

a broad central yellow field

downward

through blackto blueatthe edge, c

and

c' shade similarly

from

a yellow central field through black

and

then

have

a nar-

row

outer border of higher colors.

No segment

issimple.

Even

thec

and

c'sec- tors

which have homogeneous

parallel extinction give, in convergent fight,

an

interferencefigure likethatobtained

from

twomuscovite plates superposed at right anglestoeach other,while the

end

sectors give still

more

complicated interference figurs8, suggesting 3

mica

plates at 60° to each other.

The

acute bisectrix of allof the intergrown crystal units is perpendicular to the table.

The

simplest explanation

which

will fit these several peculiarities is that the crystals are

made

up, as before, of

an

underlying

homo-

geneouscrystalwhich, however,isnotofuniform thickness but thick- ens in all directions

from

the center. Thinning

would

produce the

same

resultbutthe thickening is actually noticeable

when

the prehn-

ite crystals are examined. This simple tapering crystal is overlain

by

layers having the arrangement of the sector pattern, figure 18.

In c

and

c' there is one overlying plate with optical directions at right angles to those of the

fundamental

crystal beneath. In a

—

^a'

and

b

—

h' three layers, the

two upper

being oriented at 90° to each other

and

45° to the underlying crystal.

/\\^.

Fig. 20.

—

Datoliteoffirstgenera tion showing acutehabit.

.ABT.2.

PETROLOGY AT

GOOSE

CREEK SHANNON. ₆₇

A

small vein in thesouth

end

of thequarryvaried

from

1 to 2 cm.

wide

and was

filled with granular fine green porous prehnite.

The

cavities are lined with brilliantlittle crystals

which

are elongated

on

the a axis

and have

the crystal habit

shown

in the orthographic

and

clinographic drawings of figure 12.

The

prism faces

gave good measurements

indicating the prismatic angle to be ^80° 59'.

The

front pinacoid also gave

good

signals but the side pinacoid (010) is etched dull.

The

base is horizontally striated as indicated.

Between

crossed nicols these crystals gave the pattern

shown

in figure 19.

Although

at first glance appearing

more

complicated, this is found to be only a variation of the structure

shown

in figure 18,

and

is capable of the

same

interpretation.

These

interpretations are applicable only to the prehnites here described

and

this

mineral seems to adopt

numerous

other con- fused intergrowths producing other effects as

shown by

the discussions of Mallard

and Emerson

abstracted in Dana's Mineralogy.

The

greenprehnite

from

oneof the sheeted veins

was

purified for analysis

and

analyzed in the

Museum

laboratory yielding theresults given in

column

1 below. In

column

2 are quoted the results obtained

on

analysis of prehnite

from Admiralty

Inlet,

and

in

column

3 the theoretical composition.

Fig.21.

—

Datolite of second generation showing moee prismatic habit by elonga- TIONON THEaAXIS

Analyses of prehnite.

68 PROCEEDINGS OF THE NATIONAL MUSEUM.

vol. GG,

The

comparison of the

above

analyses indicates that the prehnit©

from

thislocahty is ordinary in composition except in beingrather higher

than

usualin ferric-iron,alittlehigher,in fact,

than

theprehn-

ite

from Admiralty

Inlet described

by

Johnston^{^^} to

which

the vari- etal

name

^" ferriprehnite" has been applied.

That

the iron is pres- ent as ferric oxide replacing

alumina

rather than as ferrous oxidere- placing lime

was

definitely determined.

The

optical properties of the analyzed

powder

were

found

very

difficult of determination.

The

confusedopticalstructures observed in the relatively simple crystals described

above

are greatly multi- plied in the massivematerial.

Although composed

of pure prehnit

e

the

sample gave

variable refractive indices

and

^is clearly

somewhat zoned

with

some

variable constituent, probablyferricoxide, affecting the indices of the different zones.

The

average indices,

which

are the best that could be obtained, are

«=

1.635, i8=^1.640,

7=

^1.655.

The

mineral ^is biaxial positive

and

the value for

2V

varies

from

nearly 0° to about 30° or

more

with

an

average of 15°.

The

dis- persion,

r<v,

varies

from weak

to extreme,

most

intense in the grains of smallest axial angle. Confused interference figures give

extreme

crossed dispersion.

Paragenetically the prehniteisearly, definitely earlierthanstilbite^

laumontite, apophyllite,

and

calcite. In

most

specimens ^it precedes datolite but in other specimenscrusts of datolite crystals are over- lain

by

clearly later crusts of

"dumb-bell"

prehnite.

Evidence

definitelyproving

whether

this

means two

generations of prehnite or

two

of datolite

was

notfound.

From

the existence of

two

types of crystallization of the datolite, however, it is

assumed

that there are

two

generations ofthe datolite

and

only one of prehnite.

Datolite occurs in a large

number

of specimens

and

has been

assumed

to be of

two

generations because in

many

specimens it is

underlain

by

a thick crust of prehnite while in others the datolite crystalsare

more

orless covered

by

alatercrust ^ofprehnite, although bothgenerationsofdatolite

were nowhere found

inthe

same

specimen.

The

first datolite

found

in the quarry, in October, 1922,

was

only yellowish transparent granular material

on

diabase, not associated with

any

other mineral,although laumontite occurred

on

theopposite sides of the

same

specimens. This

was

identified

by

its character- istic optical properties

which

^are: Biaxial, negative (

—

_),

2V

large,

r>v

weak,

|8=1.653±.

002.

•'R.A. A. Johnston,CanadaOeol.Surv.VictoriaMemorialMuseum,Bulletin1,p. 95,1913

ART.2.

PETROLOGY AT

GOOSE

CREEK SHANNON. ₆₉ The

second lot of datolite specimens

was

obtained

from

a north- south fissure in April, 1923,

and

contained

numerous

crystals of datolite astheearliestmineraloftheveins,covered

by

later prehnite, laumontite,stilbite,

and

calcite. Theseare greenishtransparentcrys- tals of acute pyramidal habit as illustrated in figure 20.

They

average 3

mm.

in length

and

greatly resemblethecrystals of datolite

from Bergen

Hill. Entirely similar crystals line

narrow

veins later solidlyfilled with white apophyllite.

The

crystals are fairly simple in combination with the forms a (100),

m

(110),

n

(111),

and

x (102)

prominent

with smallerfaces of M ^{(Tl4), e (Tl2),}

m^

(Oil),

and

_g (012).

The

crystals of this type

which

were

measured

gave the following angles

:

Measurementsof datolite crystals, Figure 20.

Form.