l62 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 cpidote, 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 



