416 GEOLOGY. 



that of granite. In the scheme of field names recently proposed, syenite 

 is made to include all holocrystaUine rocks composed mainly of feldspar 

 of any kind, with subordinate amounts of mica, hornblende, pyroxene, 

 and other minerals, but without a noticeable amount of quartz. 



The diorites. — These embrace rocks which were crystaUized from a 

 magma still poorer in silica and the alkalies, and richer in the earthy 

 bases. In composition they closely approach the ideal average rock, 

 but usually fall a little below it in silica and the alkalies, and rise a Uttle 

 above it in the earthy bases. In current usage, diorite is defined as an 

 intimate mixture of crystals of hornblende and a plagioclase feldspar. 

 It differs from the syenite in having plagioclase feldspar instead of 

 orthoclase. By substitutions and the addition of accessory minerals, 

 the diorites graduate toward the granites and syenites on the one hand, 

 as already noted, and into gabbros on the other. 



In the scheme recently proposed, all holocrystaUine rocks in which 

 hornblende is dominant and feldspar subordinate are classed as diorites. 



The gabbros. — The name gabbro was formerly applied to a coarse- 

 grained basic rock consisting of labradorite and diallage, but the name 

 has been gradually extended until it embraces a large group of rocks 

 that have essentially the same composition as the dolerites mentioned 

 below, but are coarser in crystallization, and the crystals do not embrace 

 one another (i.e., are not ophitic). The principal minerals are plagio- 

 clase (normally labradorite) and pyroxene (normally diallage) with 

 magnetite or ilmenite. They are usually dark, heavy rocks. The 

 pearly luster of the cleavage faces of the diallage, when present, gives 

 a pecuUar sheen to a fresh surface of the rock. In the recently pro- 

 posed field names, gabbro is made to include all phanerocrystalline rocks 

 in which pyroxene predominates, attended by feldspar of any kind 

 in subordinate quantity, with or without hornblende or mica. 



The peridotites. — These stand at the basic end of the series, having 

 been formed from a magma in which the siHca was low (39-45 per cent), 

 as were also the alumina, lime, and alkaUes, but in which the magnesia 

 was relatively very high, ranging from 35 to 48 per cent. The rock 

 consists very largely of ohvine associated with pyroxene, magnetite, 

 and other very basic minerals. Little or no feldspar is present. The 

 peridotites are much less abundant than the preceding classes and repre- 

 sent a very distinctive phase of the magma in which the magnesia was 

 greatly concentrated. 



