THE MELAPHYRS AND AUGITE PORPHYRITES S6 



occurring in the form of a dike in tlie Carlingford district, 

 Ireland. 



The diabases are among the most abundant and wide-spread 

 of the so-called trap rocks, occurring in the form of dikes, in- 

 trusive sheets, and bosses. They are especially characteristic 

 of the Triassic formations of the eastern United States. It 

 should be noted, however, that many of these Triassic traps 

 have been shown to be true lava flows, and that on both litho- 

 logical and geological grounds such may be classed with the 

 basalts. 



(3) THE MELAPHYRS AND AUGITE PORPHYRITES 



The term melaphyr is used to designate a volcanic rock 

 occurring in the form of intrusive sheets and lava flows, and 

 consisting essentially of a plagioclase feldspar, augite, and 

 olivine, with free iron oxides and an amorphous of porphyry 

 base. The augite porphyrites differ in containing no olivine. 

 The rocks of this group are therefore the porphyritic, effusive 

 forms of the olivine-bearing and olivine-free diabases and 

 gabbros. 



Structure. — As above noted, they are porphyritic rocks with, 

 in their typical forms, an amorphous base, are often amygda- 

 loidal, and with a marked flow structure. 



Colors. — In colors they vary through gray or brown to nearly 

 black; often greenish through chloritic and epidotic decompo- 

 sition. 



Classification and Nomenclature. — According as olivine is 

 present or absent, they are divided primarily into melaphyrs 

 and augite porphyrites, the first bearing the same relation to 

 the olivine diabases as do the quartz porphyries to the granites, 

 or the hornblende porphyrites to the diorites, and the second 

 a similar relation to the olivine-free diabases. The augite 

 porphyrites are further divided upon structural grounds into 

 (1) diabase porphyrites which includes the varieties with holo- 

 crystalline diabase granular ground-mass of augite, iron ores, 

 and feldspars, in which are embedded porphyritic lime-soda 

 feldspars, — ^mainly labradorite, — idiomorphic augites, and at 

 times accessory hornblende and black mica; (2) spilite, which 

 includes the non-porphyritic compact, sometimes amygdaloidal 

 and decomposed forms such as are known to German petrog- 

 raphers as dichte diahase, dialase mandelstein (amygdaloid), 



