LITHOLOGY. 229 



On account of the complexity of this rock, it would be difficult to cal- 

 culate what species of feldspar is present. As the percentage of silica 

 in the rock is below that shown by the analysis of its hornblende, it cer- 

 tainly is quite basic. The chief value of these analyses is in showing 

 the ultimate composition of the original deposits from which these rocks 

 were made, and it is interesting to note that the analyses do not differ in 

 any respect from those of certain basic eruptive rocks. 



Quartz Diorite. By far the larger part of our green diorite is quartz 

 diorite. In some specimens, quartz is present only in the most insig- 

 nificant amount, but in others its quantity is increased till it greatly pre- 

 ponderates, and the dividing line between diorite and amphibolite is 

 reached. As distinguished from the quartz-free diorites, these rocks 

 appear to be more fresh and undecomposed ; chlorite is not so common, 

 and augite is entirely absent. Their color is darker, and the hornblendic 

 character is more evident to the unaided eye. I have selected for illus- 

 tration a quartz diorite from Lisbon. The rock is dark green in color, 

 but is spotted with white spots, which can be in part recognized as feld- 

 spar by the cleavage. In a thin section, the hornblende appears in large 

 fibrous masses, in bunches of radiating needles, and in minute separate 

 crystals. This hornblende assumes the most brilliant colors in polarized 

 light. Its appearance is represented in Fig. 4 on PI. 12. This will make 

 plain what I have said in reference to the condition of the hornblende in 

 metamorphic diorites, as contrasted with the eruptive rocks. 



The triclinic feldspar appears in quite large crystals, which mostly show 

 wide bands in polarized light. What species it is, one cannot say. From 

 the siliceous nature of the rock, it might be supposed to be oligoclase or 

 albite, and this has commonly been assumed as the fact ; but in this case 

 the conclusion would scarcely be justified, for the region is one of low 

 metamorphism ; and under such circumstances, reasoning from the proba- 

 ble conditions of chemical stability is of doubtful weight. It is enough 

 to call it plagioclase. The quartz fills up the interspaces, and sits in all 

 the little corners. Grains of titanic iron and bits of biotite are present. 



In a specimen of this diorite from Northumberland the feldspar is 

 entirely altered into an aggregate in which epidote is prominent, Sphene 

 is also quite abundant, 



A specimen from Hanover (boulder) is typical of many others. It is 



