SOME NORTH-OF-ENGLAND DYKES. Don 
decomposition, and is usually found only in those slides in which the 
augites have been replaced by calcite. The irregular crystalline 
plates occur in comparatively unaltered rocks, and they are here 
seen to be almost invariably associated with quartz. Both quartz 
and calcite also occur in thin veins. Small spherical amygdules 
containing calcite and chalcedony occur in certain localities, and 
especially at Bowles Beck, where my attention was first called to 
them by Mr. James T’Anson. 
Pyrites.—This mineral occurs abundantly in certain localities, 
but is not distributed throughout the rock with any definite regu- 
larity. It owes its origin, no doubt, to local causes. Under the 
microscope it appears as irregular ragged plates, the true nature of 
which can, of course, only be recognized by reflected light. 
The Interstitial Matter.—This is in some respects the most inter- 
esting portion of the rock. It forms, as a rule, about one half of 
the ground-mass, and presents a number of remarkable structural 
variations. Although giving, in the majority of cases, a decided 
reaction under polarized light, 1t consists of a number of imperfectly 
individualized substances. In order to study the characters of this 
substance, it is absolutely necessary to have the thinnest possible 
sections and to use a magnifying power of not less than 500 diame- 
ters. We will consider first of all the case of a somewhat exceptional 
variety of the rock, which occurs at Armathwaite. The ultimate 
base is a clear isotropic glass containing a large number of extremely 
minute brownish granules. With a magnifying power of 500 
diameters the smallest granules appear as mere specks, while the 
larger ones have frequently a globular or elliptical form. Clusters 
of granules may frequently be seen, and these appear to show a 
tendency to a linear arrangement, The granules are without 
action on polarized light. They belong to the globulites of Vogel- 
sang (Die Krystalliten). In addition to the globulites, there are 
also long, more or less conical needles of the same substance. The 
thin ends of these needles are usually clear and transparent, and 
the external surfaces smooth ; but the thick ends are rough, and 
evidently made up of globulites which have not entirely lost their 
individuality in the attempt to form these needle-like bodies. The 
bodies in question give no reaction with polarized light, and they 
correspond therefore with the longulites of Vogelsang. The smaller 
ends of the longulites rarely, if ever, lie free in the glassy base, but 
are in contact with minute augite granules, around which they are 
arranged in a rude kind of radial manner (see Pl. XII. fig. 2a). 
In short the smaller augite grains may be described as bristling with 
longulites. It is worthy of note that the felspars of the ground-mass 
appear to have exercised no influence whatever on the grouping of 
the globulites and longulites. 
The conical needles which occur in a slag figured by Vogelsang 
(Die Krystalliten, pl. ii.) are described by him as having been 
formed by a double process of growth, the pointed end having been 
produced by the coming together of the very smallest spherical 
elements (“ die kleinsten constituirenden Kugelelemente ”), and the 
