82 BOCKS rOEMEB THROUGH IGNEOUS AGENCIES 



the feldspars occur in broad, irregular forms surrounding the 

 more perfectly formed pyroxenes. Transitions into diabase struc- 

 ture are not uncommon. Through a molecular change of the 

 pyroxenic constituent, the gabbros pass into diorites, as do also 

 the diabases. 



Colors. — The prevailing colors are gray to nearly black; 

 sometimes greenish through decomposition. 



Classification. — The rocks of this group are divided into (1) 

 the true gablros — that is, plagioclase-diallage rocks — and (2) 

 noriteSy or plagioclase-bronzite and hypersthene rocks. Both 

 varieties are further subdivided according to the presence or 

 absence of olivine. "We then have : 



True gabbro = Plagioelase + diallage. 

 Olivine gabbro = Plagioelase + diallage and olivine. 

 Norite = Plagioelase + byperstbene or bronzite. 



Olivine norite = Plagioelase + hyperstbene and olivine. 



Nearly all gabbros contain more or less rhombic pyroxene, and 

 hence pass by gradual transitions into the norites. Through 

 a diminution in the proportion of feldspar they pass into the 

 peridotites, and a like diminution in the proportion of pyroxene 

 gives rise to the so-called forellenstein, Hyperite is the name 

 given, by Tornebohm, to a rock intermediate between normal 

 gabbro and norite. Anorthosite is the name given to the granular 

 varieties poor or quite lacking in pyroxenes. 



(2) THE DIABASES 



Diabase, from the Greek word Sta^Sao-t^, a passing over; so 

 called by Brongniart because the rock passes by insensible grada- 

 tions into diorite. 



Chemical Composition. — The table on page 83 shows the aver- 

 age range in composition of (I and II) the plutonic diabase and 

 (III, IV, V, and VI) the effusive forms melaphyr and basalt. 



Mineral Composition. — The essential constituents of diabase 

 are plagioelase feldspar and augite, with nearly always mag- 

 netite and apatite in microscopic proportions. The common 

 accessories are hornblende, black mica, olivine, enstatite, hyper- 

 sthene, orthoclase, quartz, and titanic iron. Caleite, chlorite, 

 hornblende, and serpentine are common as products of altera- 

 tion. Through a molecular change known as uralitization the 



