202 IGNEOUS ROCKS 



and many localities in the Connecticut valley, New Jersey, Mary- 

 land, Virginia, and North Carolina, are diabase. 



Gabbro is a term which is now used comprehensively to include 

 the coarse-grained, plutonic phases of the various basaltic rocks, 

 which are typically composed of plagioclase and pyroxene. Olivine 

 gabbro and hornblende gabbro are names that explain themselves. 

 No rite, or hypers thene gabbro, contains orthorhombic pyroxene. 

 Anorthosite is nearly pure labradorite in large crystals, with little 

 or no pyroxene : great masses of it occur in Canada and the 

 Adirondack Mountains of New York. 



Note. — Since the foregoing paragraphs were written, Professor Pirsson has 

 described a group of basaltic rocks which have large, transparent phenocrysts 

 of analcite with pyroxene, olivine, and other dark silicates embedded. These 

 rocks are called Monchiquites. 



V. The Ultrabasic Rocks 



These rocks have no felspars, and in most of them the quantity 

 of silica is below 45 %, while that of magnesia is from 35 to 48 °j ', 

 they are composed almost entirely of ferro-magnesian minerals. 



Limburgite is made up of crystals of augite, olivine, and mag- 

 netite, embedded in a glassy ground mass. 



Augitite is a similar rock, but without olivine. 



Pyroxenite is a holocrystalline, plutonic rock composed of one 

 or more varieties of pyroxene. 



Hornblendite is a similar rock made up of hornblende. 



The Peridotites are likewise plutonic rocks which are principally 

 composed of olivine, with iron ore and some of the pyroxenes or 

 hornblende. 



The Serpentine Rocks are products of decomposition, and many 

 of them have been formed from the peridotites, though some 

 are derived from augitic rocks, such as gabbro, and others from 

 hornblendic rocks. In rarer instances they have arisen from the 

 alteration of acid rocks. 



