1864.] T. STERRY HUNT ON LITHOLOGY. 33 



to class such anortliosite rocks as contain a black ferruginous 

 pyroxene or augite. These rocks, which are sometimes coarsely 

 granular or granitoid in their structure, pass into fine-grained or 

 compact varieties, which are distinguished by the names of aname- 

 site and basalt. To these latter varieties belong a great part of the 

 greenstone-traps, although in rocks of this texture it is often 

 impossible to determine whether it is hornblende or pyroxene 

 which is mingled with the feldspar. Olivine in grains or crystals 

 frequently occurs both in the fine-grained basaltic dolerites and 

 the granitoid varieties, giving rise by its predominance to what is 

 called peridotite. Some fine-grained dolerites are porphyrilic 

 from the presence of black cleavable augite crystals, forming an 

 augite-porphyry. Finely disseminated carbonates of lime and oxyd 

 of iron are occasionally present in these rocks to the extent of 

 twenty per cent., and even more. In like manner, magnetite and 

 ilmenite, which are often associated, may constitute several hun- 

 dredths of the mass. Many fine-grained greenstones contain, like 

 phonolite, large portions of some zeolitic mineral, and they often 

 abound in chlorite. The pyroxene in these rocks is sometimes 

 replaced by a highly basic silicate. Some varieties of what has 

 been called diallage may be represented as an aluminiferous pyrox- 

 ene 'plus a hydrate of magnesia. At other times a mineral 

 approaching in composition to a ferruginous chlorite (frequently 

 amorphous) enters into the composition of these anorthosites, and 

 even in some cases appears to replace altogether the pyroxene or 

 the hornblende, constituting an aberrant form of diorite or of 

 diabase, which is not uncommon among greenstones, and for which 

 a distinctive name is needed. See on this point Geology of Canada, 

 pp. 469, 605, and the remarks on melaphyre below. 



The finer-grained dolerites are often cellular, giving rise to 

 amygdaloids, whose cavities are generally filled with calcite, 

 quartz, or some zeolitic minerals. To these amygdaloids the name 

 of spilite is sometimes given. Earthy varieties of basalt, which are 

 frequently the result of partial decomposition, constitute the wacke 

 of some writers. It is doubtful how far many of these spilites and 

 wackes have a claim to be considered as crystalline rocks, inas- 

 much as they appear in very many cases to be nothing more than 

 aqueous sediments accumulated under ordinary conditions, or per- 

 Jiaps in some cases derived from volcanic ash or volcanic mud. As 

 the other extreme of this series of rocks we may notice that dole- 



Vol. I. . Jfo. 1. 



