10 MR W. F. P. M'LINTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



aggregate of scolccite and prehnite. Cubes of pyrites have also been observed 

 enclosed in massive prclmitc. 



One specimen of special interest shows a hollow vesicle lined with, apparently, 

 dull, opaque icositetrahedra of analcite upon which are seated crystals of pale 

 green epidote ; the icositetrahedra when crushed and examined under the microscope 

 are seen to consist of albite which has replaced the zeolite. 



The typical vesicles filled with fibrous scolecite sometimes reach a phenomenal 

 size, the largest one found measuring 15 cm. along its greatest length. They are 

 usually closely packed with scolecite, terminated crystals of which are exceedingly 

 rare: a few have been found, however, showing the combination b(010), ?n(ll0), 

 cZ(101), o(lll). In the simplest cases the scolecite is seated upon a layer of chlorite, 

 but such specimens are rather exceptional. A more common association is (beginning 

 from the wall of the amygdale) chlorite with deep green epidote, then a layer of 

 crystalline albite, white or pale pink in colour, and of variable thickness, and, lastly, 

 scolecite which fills the vesicle ; a layer of prehnite sometimes intervenes between 

 the scolecite and albite. Calcite occurs fairly frequently in crystalline, much-cleaved 

 masses enclosed in the scolecite near the junction with the underlying minerals. 



Epidote is exceedingly abundant. When it occurs with scolecite it is always 

 found near the walls of the vesicle in intimate association with chlorite. It forms 

 groups of dark green radiating crystals which sometimes spear the scolecite and are 

 occasionally completely enclosed by it. In a second type of vesicle, where scolecite 

 is absent, it occurs in groups of divergent crystals seated upon albite, whilst in a 

 third type the amygdale is packed with massive chlorite, in the centre of which is a 

 kernel of fibrous epidote. 



Chlorite is universally present in the cavities and shows various interesting 

 features, some of which have an important bearing on the question of the date of 

 formation of the vesicle-minerals. It was obviously one of the first to be deposited, 

 and forms a dark green, compact, and massive layer lining the walls of the amygdale. 

 This layer varies considerably in thickness : in some cases, especially when the 

 vesicle is small, it fills the cavity ; in others it forms a thin and patchy priming for 

 the overlying minerals, which may be scolecite, albite. epidote, or prehnite. In many 

 cases the chlorite is quite homogeneous, but it often contains crystals and grains 

 of deep green epidote, whilst in a few specimens acicular crystals of black augite, 

 measuring up to - 4 cm. in length, can be detected. These crystals occur usually 

 at or near the junction of the chlorite with the rock, but occasionally they are found 

 well within the chlorite layer and even wandering into the overlying epidote, 

 albite, and prehnite. Sometimes they are arranged radially to the walls of the 

 vesicle to which they are often attached, but in many instances they lie tangentially 

 to the boundary and have no visible point of attachment to the rock. On one 

 specimen the layer of chlorite and augite can be seen to connect with a vein packed 

 with similar material which pierces the lava. The crystals of augite in the centre 



