78 LITHOLOGICAL GEOLOGY. 



2. Hornblende and Augite Series. 



In the rocks of this series magnetite or titanic iron is almost always present, as 

 well as apatite. Some augitic kinds contain native iron in grains and rarely in masses. 



The sections are: (1) The rocks having hornblende as a prominent constituent, in- 

 cluding syenyte, dioryte (with propylyte), andesyte; (2) The rocks containing augite, 

 with little or no hornblende, including augite-andesyte, doleryte (with diabase), am- 

 phigenyte, nephelinyte, and some obsidian. The rocks of the second division, except- 

 ing the first mentioned, are basic rocks. The term trap was early applied in Sweden 

 (from trappa, step) to the compact columnar doleryte or basalt. The augitic kinds 

 are often amygdaloidal. 



1. Syenyte and Quartz- syenyte. — See p. 73 for description. 



2. Dioryte and Quartz-dioryte. — See p. 73. Piopylyte and quartz-propylyte 

 have the same constitution. The former is the prevailing igneous rock of the Washoe 

 district (vicinity of the Comstock lode), in Nevada; it is a grayish-green rock, yield- 

 ing, on analysis, 64 to 66 per cent, of silica, and containing, along with oligoclase, 

 hornblende, disseminated in minute points, and rarely also biotite. 



3. Andesyte and Quartz-andesyte. — Andesyte is similar to the last, but affords 

 about 60 per cent, of silica. The original andesyte from the Andes contains the feldspar 

 andesite along with hornblende, which gives silica 57 to 60 per cent., while oligoclase 

 gives 61 to 64 per cent. As in the preceding, the hornblende is sometimes changed to 

 chlorite. 



Quartz- Andesyte, or Dacyte, is a quartz-bearing variety. Both kinds occur in the 

 Washoe district. 



4. Variolyte (of Durance). — Consists of concretions of oligoclase and green 

 hornblende, with some grains of pyroxene, in a fine-grained partly amorphous base, 

 which is made up of labradorite, green and black hornblende, and an ill defined ma- 

 terial serpentine-like in aspect, and partly fluidal in texture. The nodular contains 

 57 per cent, of silica, and the base 45 per cent. 



5. Corsite. — A granitoid rock, consisting chiefly of anorthite and hornblende, with 

 some quartz, and biotite from Corsica. 



6. Augite-Andesyte. — Contains the same triclinic feldspar as andesyte, but au- 

 gite is present in place of hornblende. Amount of silica obtained in analysis about 

 55 to 58 per cent. Texture crystalline-granular to aphanitic; colors dark-gray to green- 

 ish-black and brownish-black. Sp. gr. 2-65-2-90. 



Varieties. — There are two series: A. Ordinary, that is, without chrysolite, or only 

 in traces. B. Chrysolilic, chrysolite being in disseminated grains or crystals. Under 

 each there are other varieties: a. anhydrous; b. hydrous, or chloritic, and feeble in 

 lustre ; and c amygdaloidal, as well as chloritic. Again, each of these varieties may 

 be porphyritic. To the hydrous rock, and especially the chrysolitic, the term mela- 

 phyre is sometimes applied. 



Ophite has nearly the composition of augite-andesyte, but contains diallage, and 

 more resembles euphotide. Sometimes it contains serpentine disseminated among the 

 other minerals. Some authors have made it metamorphic. There is also an oligoclase- 

 ophite, containing much quartz. 



7. Doleryte (Basalt). — Chief constituents, labradorite, or anorthite, and augite, 

 with magnetite. Amount of silica yielded on analysis usually 47 to 52 per cent. Tex- 

 ture crystalline-granular to aphanitic. Colors dark grayish to greenish-black and 

 brownish-black. Sp. gr. 2-75-3-1. 



Varieties. — There are two series : A. Ordinary ; B. Chrysolitic ; and for the lat- 

 ter the name peridotyte has been used. Each occurs: a. anhydrous; b. hydrous, or 

 chloritic, of feeble lustre ; c. amygdaloidal, as well as chloritic ; d. vesicular, or scori- 

 aceous, as in doleritic lavas. Again, each of these varieties may be porphyritic. A 

 coarse-granular kind, having the pyroxene foliated, is sometimes called gabbro. 



This basic rock, doleryte, is what is ordinarily called trap ; and sometimes, also, 

 basalt. The name, anamesite, has been used for an aphanitic kind, but is unnecessary. 

 The term diabase is sometimes applied to dolerytes older than Tertiary. It was for- 

 merly supposed that diabase, or the pre-Tertiary rock, differed from doleryte in being 

 chloritic, and afterwards in not containing glassy particles : but neither distinction holds 



