638 §. ALLPORT ON THE DIORITES OF 
intrusion cannot be precisely determined, it is at least certain that 
it was previous to the deposition of the Triassic rocks, for in an 
old quarry near Marston Jabet the Lower Keuper sandstone lies 
horizontally on the upturned edges of the Carboniferous shales and 
included trap. 
Tue Microscoric StRUCTURE OF THE WARWICKSHIRE DiIoRITES. 
An examination of numerous specimens from the different masses 
shows clearly that, as a group, these rocks must be regarded as 
diorites, the characteristic constituents being a triclinic felspar and 
hornblende ; these two minerals together with magnetite and apatite 
are invariably present, and in addition a little orthoclase is seldom 
absent. Although many of the specimens examined are ordinary 
diorites, there are occasionally such wide departures from the normal 
type, that, from a mineralogical point of view, some of them must 
clearly be classed with rocks not hitherto observed among the older 
eruptive series. On the whole, it would perhaps be difficult to point 
to a group of rocks presenting more interesting varieties of com- 
position and texture, or which afford more instructive examples of 
extensive alteration. 
Quarry near Marston Jabet.—A finely crystalline specimen, very 
like a basalt in external appearance, contains an immense number 
of small hornblende crystals in a felspathic base, both minerals 
being nearly or quite unaltered. The clear brown crystals of horn- 
blende are unusually well developed, and as they lie in all directions 
a single thin slice affords many excellent transverse and longitudinal 
sections of the prism. One rarely meets with an augitic or horn- 
blendic rock containing such an assemblage of well-formed crystals. 
They are thickly set in a clear matrix of triclinic felspar, and 
interspersed among them are rather numerous grains of magnetite, 
with here and there a few needles of apatite. The only indication 
of alteration is a little calcite in the matrix; no other minerals are 
resent. 
The three following specimens, selected from a number collected 
in the same quarry as the last, may be taken to represent the more 
general character of the rocks of the district; and they possess a 
special interest, as they afford excellent examples of successive stages 
of alteration. The first example is rather coarsely crystalline in 
texture, and the original constituents are very well preserved; the 
plagioclase is generally clear, and exhibits well its characteristic 
twin striation. The hornblende is of a clear brown colour, for the 
most part quite unaltered, and presents its ordinary optical and 
physical characters ; it is much fissured, and occasionally contains 
80 many cavities that the crystals are little more than skeletons ; it 
also encloses many grains of magnetite, and the latter are also rather 
thickly disseminated through the mass. Long hexagonal needles of 
apatite are rather abundant; and, lastly, there was a glassy or felsitic 
ground-mass in which the crystallized constituents were set. This 
