l60 MINERALOGY AND LITHOLOGY. 



scope reveals the frequent presence of the same accessory ingredients, 

 such as biotite, apatite, chlorite, pyrite, and calcite. 



The study of these rocks leads us through the same channels as be- 

 fore. We have to consider wide differences in the composition of the 

 original rocks, and wide differences that have resulted from the decay 

 which long ages have induced. It may be stated, however, that the dif- 

 ferences in the original compositions of our diorites are greater than of 

 our diabases, while the more stable nature of hornblende has preserved 

 the diorites from such universal alteration as characterizes our diabase. 



Diorites may originate from an alteration of the augite of diabase into 

 hornblende, or from the original crystallization of hornblende. Mr. Al- 

 port states, that the first process of formation is quite general in the 

 English rocks at Landsend, and that all stages of the process of change 

 are easily found. I have spoken of incipient change of this kind in de- 

 scribing certain kinds of diabase; but our eruptive diorites usually con- 

 tain microscopic or macroscopic hornblende crystals, which are quite 

 well crystallized in the form characteristic of the species, — and hence we 

 are dealing in this respect with primary formations. 



Our diorites, being mostly porphyritic, belong to the class of rocks 

 which, by some Germans, are called porphyrite. I think no one will ob- 

 ject to the simple inversion of their terms which I employ. Porphyritic 

 diorite means the same as diorite porphyrite. The ground mass of 

 these porphyritic diorites is wholly crystalline, and though large crystals 

 are developed in it, it is diorite still. 



There seem to be two quite distinct types of this rock in New Hamp- 

 shire, one of which is very basic in composition, and only hornblende is 

 porphyritically developed in it, while the latter contains more silica; and 

 both hornblende and plagioclase are conspicuously developed in the 

 black compact ground mass. The first variety contains a triclinic feld- 

 spar, which cannot be identified with any certainty, and we will call the 

 rock merely basic diorite. The second variety contains andesite or oligo- 

 clase, and we will speak of it as plagioclase diorite. 



Basic Diorite. (Porphyritic Diorite.) This is a rock which has for a 

 ground mass an aggregate of crystals of triclinic feldspar, hornblende, 

 biotite, and magnetite or titanic iron, with usually some chlorite and 

 apatite. In it well formed crystals of hornblende are developed, and at 



