SOME NORTH-OF-ENGLAND DYKES. 219 
or absence of oligoclase or albite. The researches of M. Fouqué on 
the mineralogical composition of the Santorin augite-andesite lava 
of 1866, led him to the conclusion that the porphyritic felspars of 
that rock were mainly labradorite, but that they comprised also 
anorthite, sanidine, and oligoclase, and that the felspars of the 
ground-mass were albite and oligoclase. It is highly probable’ 
that the rock we are considering, which appears to be a fairly 
typical augite-andesite, though not so rich in silica as the lava of 
Santorin, also contains a mixture of different felspars. 
In the description of the remaining felspathic constituents of the 
ground-mass it will be convenient to begin with the microlites. 
These may be either straight or curved. The majority of them lie 
between the following dimensions :—width -002m. to ‘004 mm. ; 
length ‘(06 mm. to -°35 mm. As a rule, these microlites are not 
sharply defined from the interstitial matter; thus, the interstitial 
matter of the rock which occurs at the old quarry at Barwick, on 
the right bank of the Tees, near Preston, shows under a high power 
(270 diameters) a sort of mottled appearance due to the irregular 
interlacing of darker and lighter portions. In places these lighter 
portions assume a linear form, and take on the characters of the 
microlites (see Pl. XII. fig. 40). Curved microlites are common in 
the same rock ; they are sometimes clustered together so as to give 
rise to curious plume-like forms. The straight microlites extinguish 
in the direction of their length. The curved microlites have the same 
optical properties, and do not therefore extinguish simultaneously 
in any one position, only that portion of any particular microlite 
appearing dark which happens to lie parallel with one of the two 
vibration-planes. Not unfrequently the microlites show a tendency 
to unite with each other at definite angles; and this leads to the 
building-up of curious skeleton-like forms. The rectangular mode 
of junction is the most common. When the primary microlite is 
curved, then the secondary microlites converge like the spokes of a 
wheel. . 
The skeleton felspars (see Pl. XII. fig. 4 a) constitute an inter- 
mediate group between the microlites and the more or less perfect 
felspars, and they exhibit a very great diversity of form. Commencing 
with those which consist of a primary microlite to which secondary 
microlites have attached themselves at right angles, we may pass, by 
means of those which consist of two parallel microlites, joined, either 
at their centres or at their ends, by transverse microlites, to broad 
rectangular felspar sections, bounded by sharp straight edges, or else 
running out at their ends into acicular microlites. These skeleton 
forms are best studied in sections from the Barwick and Arma- 
thwaite quarries. I have observed somewhat similar forms in the 
dolerite of Sababurg, in Hesse, and in that of Dalmahoy Hill, near 
Edinburgh, also in some of the other dykes to be hereafter described. 
Boricky has described similar forms in the melaphyres of Bohemia 
(Petrographische Studien an den Melaphyrgesteinen Bohmens, p. 9, 
pled. fie, 7). 
Augite.—This mineral, like the felspar, occurs in two conditions, 
