TATE: YURKSHIKE PETROLOGY. 
815 
little bunches and wisps within the interspaces, form one of its most 
striking features (fig. 40). Clear interstitial spaces under higher 
magnification may he resolved into a little residual isotropic glass, in 
which lie intimately iyterwoven feathery and felted bundles of 
aciciilar microlites, the m'lcrol'ithenjilz of the Germans (fig. 4). The 
somewhat longer microlites marshalled in radial symmetry usually 
based upon a porpli3Titic mica-plate (fig. 42), are anisotropic and of 
a salmon-coloured tint. Imbedded in this micro-crystalline basis 
lies a profusion of mica-plates in which the recun-ence of phase is 
most markedly indicated b}- the deeply eroded dark-brown fiag- 
ments of the older generation, contrasted side by side with the com- 
paratively fresh and pale aspect of the younger series. In place of 
recognizable hornblende, we find in this, as in the uppermost dyke, 
pale-green patches of a dimly dichroic and faintly polarising chloritoid 
mineral, in many instances bordered by dwarfed mica-plates. There 
is also a considerable sprinkling of chocolate-coloured titaniferous 
magnetite in grains and skeleton crystals (fig. 14), portions of which 
have been transformed into small nearly colourless crystals of sphene 
polarising in firm tints. Pyrite with a brassy glint by reflected 
light ; calcite grains and quartz blebs lie scattered through the 
interstitial matter. 
This rock with its plentiful supply of brown mica may at sight 
be distinguished from all the Lamproph}Tes yet examined. One of 
two boulders pointed out to the writer . some three years ago at 
Ingleton, was at once assigned to this parent rock, an identification 
which subsequent microscopic comparison has confirmed. 
None of the remaining Dentdale mica-traps call for .special 
examination. Two of the exposures on the opposing slope are allied 
to the typical examples selected from Helmsgill for description. The 
other rocks belonging to this gTOup are too much decomposed for 
profitable study or identification. 
Specimens of these rocks, as they are collected, are being analysed 
in the Chemical Laboratory of the Yorkshire College, under the 
personal supervision of Mr. Bothamley, F.I.C., and the results of this 
work we hope to publish in a subsequent paper. 
