162 MINERALOGY AND LITHOLOGY. 
filled with the same aggregate of minute crystals as that which forms 
the ground mass. This enclosure of material is usually indicative of rapid 
growth. A section of a crystal of this nature is shown in Fig. 5 on Pl. 9. 
It is drawn as it appears in ordinary light, and even there the twinned 
sides of the crystal can be distinguished from one another by a slight 
difference in their shade. To the left of the large crystal is a hexagonal 
crystal, probably of titanic iron, though such a section could be cut from 
a dodecahedron of magnetite. Mr. Zirkel, when he found crystals ex- 
actly like these in the basalt of the Lacher See, supposed them to be of 
titanic iron. Inside the hornblende crystal, a crystal of this iron oxide 
of some size has developed among the other finer ingredients of the rock. 
With the aid of polarized light, the clear spots in the base are found to 
be feldspar crystals, the outlines of which are hidden in the ground 
mass. Some clear spots are cavities filled with calcite, and which 
evidently were formed by the rotting away of some mineral. 
In another specimen from Campton falls the feldspar becomes more 
prominent, and the rock consequently lighter in color; the mica de- 
creases in quantity, and fine crystals of hornblende take its place in the 
ground mass. This ground mass is so coarse as almost to destroy the 
porphyritic character of the rock. Apatite needles are abundant, and the 
iron oxide appears to be crystallized magnetite. 
A specimen of diorite, from boulders in North Lisbon thought to have 
been derived from dykes cutting porphyritic gneiss, shows the interesting 
feature of the well defined outlines of hornblende and augite crystals 
associated together. They were plainly simultaneous and original for- 
mations. This rock contains more feldspar, and the ground mass is con- 
sequently quite light in color, but the feldspar is so decomposed that in 
thin sections it is only translucent, and its optical properties are obscure. 
Embedded in this coarse ground mass, black hornblende crystals are 
prominent, and in thin sections they appear to be perfectly fresh and 
undecomposed, and in part very well formed. The augite which first 
becomes visible when the microscope is employed was originally very 
perfectly crystallized, but now it is nearly all decomposed, and its place is 
filled with an aggregate of epidote, calcite, chlorite, &c., but still some of 
its original structural lines are preserved in the new products, and some 
crystals still possess an augite core. One section that I have examined 
