216 J. J. He TEALL—-PETROLOGICAL NOTES ON 
that portion of the rock which cannot be resolved into distinct 
crystals or crystalline grains by the aid of the microscope; it may 
be glassy, micro-felsitic (Rosenbusch), erypto- or even micro-crystal- 
line. It usually gives adecided reaction with polarized light, a true 
glass having been distinctly recognized only in one instance, viz. in 
that of a specimen from Armathwaite. The secondary constituents 
comprise (a) quartz, (b) calcite, (¢) chloritic alteration-products, (d) 
limonite, (¢) pyrites. Some of the quartz may be original. 
In order to form some notion of the relative importance of the 
different constituents of the ground-mass, I selected two fairly 
characteristic photographs, and, having cut out the portions repre- 
senting the different constituents, weighed them separately. The 
following is the percentage composition of the rock as thus deter- 
mined :—-Augite and magnetite 11°4 per cent., small felspars 29-1 
per cent., interstitial matter 59°5 per cent. It is difficult to cut out 
the parts exactly, so that a liberal margin for error must be allowed. 
It must also be remembered that the interstitial matter contains 
microlites of augite and felspar, which cannot be cut out and weighed 
separately. The magnetite was taken with the augite because it 
was found impossible to separate the two in the photograph; mag- 
netite, however, forms only a very small proportion of the entire mass 
of the rock. The relative abundance of the different constituents is 
a fairly constant feature of this and also of the other dykes to be 
described; so that the application of this method, originally due to 
Dr. Sorby, I believe, becomes of considerable importance. 
Large Felspars—These occur, as already stated, in a tolerably 
constant proportion, and give the rock of this dyke a macroscopic 
character which enables one to recognize it at once wherever it 
occurs. Sections more or less parallel to the brachypinakoid appear 
as large flat plates, having tolerably equal dimensions in the different 
directions, and polarize in a uniform tint, or else show striations 
due to twinning on what may be assumed to be the pericline type. 
This twinning is very irregular as regards the length and breadth 
of the lamelle. Sections out of the zone of the brachydiagonal axis 
are frequently much longer than broad, and they show the striation 
due to twinning on the albite plan. This twinning is also irregular 
so far as the width of the bands is concerned. Sometimes the 
section is divided into two well-marked halves: but when this is 
the case each half usually contains a few very narrow bands which 
extinguish simultaneously with the other half, thus showing that 
the crystals are twinned on the albite and not on the Carlsbad 
plan, as might at first sight be supposed. The large felspars are 
frequently in the condition of fragments ; and sometimes a crystal 
may be seen to be fractured and to have had its parts slightly 
displaced, but not actually separated from one another. The frag- 
mentary state of many of the large felspars, as also the fact that 
they occur in the same condition at the sides and in the centre of 
the dyke, proves that they have not been formed zn sztu, but have 
been brought into their present position from below. 
It is difficult to obtain cleavage-flakes of these felspars for optical 
She Den 5 
