CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
135 
mass are evidently the result of an early consolidation, unless where they 
are survivals from fragments of older porphyritic rocks which have been 
enveloped and partially dissolved in the dykes. They are often cracked, 
penetrated by the groundmass, or even broken into fragments, and have 
corroded borders.. They sometimes include portions of the groundmass, and 
present the zonal growth structure in great perfection. The small felspars 
of the groundmass, on the other hand, are as obviously the result of a 
later crystallization, for they vary in size and crystallographic development 
according to their position in the dyke. Those from the centre are often 
in well-formed crystals, which sometimes pass round their borders into 
aeicular microlites. Those in the marginal parts of the dyke occur chiefly 
in the form of these microlites, forming the felted aggregate so character- 
istic ot the andesites. Curious skeleton forms, composed of aggregates of 
microlites, connect the latter with the more completely developed crystals, 
and illustrate the mode of crystallization of the felspathic constituents of 
the dykes . 1 
The pyroxene is probably in most cases monoclinic (black or common 
augite), but is sometimes rhombic (usually enstatite, less frequently perhaps 
hypersthene). It occurs in ( a ) well-developed crystals, ( 6 ) crystalline 
masses with some of the faces of the crystals developed, (c) granular 
aggregates which polarise in one plane, (d) separate granules and 
microscopic microlites, which may be spherical (globulites) or oblong 
(longulites). 
The black iron-oxide is sometimes magnetite, sometimes ilmenite, 01- 
other titaniferous ore. Apatite not infrequently occurs among the original 
constituents. Olivine is entirely absent from most of the large solitary 
dykes, especially at a distance from the great volcanic centres, and no serpen- 
tinous matter remains to indicate that it was ever present in them. But it is 
to be met with in numerous basalt-dykes in the volcanic areas, either in 
sparsely scattered or in tolerably abundant crystals. Biotite occasionally 
appears. Among the secondary products, calcite and pyrites are doubtless 
the most common. To these must be added quartz, chalcedony and various 
zeolitic substances, besides the aggregates which result from the decom- 
position of the ferro-magnesian constituents and the oxidation of the 
ferrous oxides. 
In many dykes there is little or no interstitial matter between the 
crystalline constituents of the groundmass. In others this matter amounts 
to a half or more of the whole composition, and from such cases a series of 
giadations may be traced into a complete glass containing only the rudi- 
mentary forms of crystals (globulites, longulites, etc.), with scattered 
porphyritic crystals of an earlier consolidation. The process of the dis- 
appearance of this original glass may be admirably studied in many dykes. 
At the outer wall, the glass remains nearly as it was when contact with 
the cold walls of the fissure solidified it. From that external vitreous layer 
the successive devitrification products and crystalline growths may be 
■ 1 See Mr. Teall’s excellent description of the Cleveland dyke, in the paper above cited. 
