406 



one and the same section may show both the microlitic and ophitic 

 structures. The ophitic structure could not be perfectly developed 

 with labradorite. 



It appears, therefore, from the above experiments that many 

 igneous rocks can be produced by pure igneous fusion without the 

 intervention of water in any kind of way. Not only are the positive 

 results obtained by Messrs. FOUQUE and LEVY of the 'greatest value, 

 but their negative results are also of great interest. All attempts to 

 produce rocks containing quartz, orthoclase, albite, white mica, black 

 mica and hornblende have failed. A mixture of 4 parts of microcline 

 and 4*8 of biotite yielded on cooling a crystalline mass composed of 

 leucite, olivine, melilite and magnetite. Fused wernerite crystallized 

 on cooling as labradorite, and a mixture of wernerite and hornblende 

 gave rise to a labradorite-augite rock. A mixture of oligoclase and 

 hornblende gave rise to an oligoclase-augite rock (augite-andesite). 



The association of orthoclase and quartz, so characteristic of many 

 igneous rocks, has never been produced by pure igneous fusion, but 

 it has been obtained by heating the constituents of these minerals in 

 a closed vessel in the presence of alkaline water. 



In concluding this portion of our subject let us take a general 

 and more or less speculative view of the phenomena attending 

 the consolidation of igneous magmas. When the consolidation is 

 effected under what may be termed plutonic conditions, that is, 

 under a pressure vastly in excess of that due to the atmosphere, 

 a holo-crystalline rock of granitic structure is produced. The 

 first-formed constituents are idiomorphic, but the later constituents, 

 which generally make up the main mass of the rock (e.g., quartz 

 and orthoclase in granite ; plagioclase and diallage in gabbro) 

 are allotriomorphic. Plutonic masses may be homogeneous, that is, 

 the different constituents may be mixed in fairly uniform propor- 

 tions; or they may be heterogeneous, that is, the different con- 

 stituents may be mixed in different proportions in different parts. 

 This heterogeneity is represented by the so-called concretionary 

 patches (diorite in granite) by contemporaneous veins (granite and 

 granophyre in diorite, eurite in granite, troctolite in peridotite) and 

 sometimes by a gradual passage from one kind of rock to another 

 in the same mass (gabbro or augite-diorite into granophyre, as in 

 Carrock Fell). In some cases one kind of plutonic rock, such as 

 granite, is seen to vein another kind of rock such as diorite in the 

 most intricate manner. Such cases may be regarded as exaggerated 

 instances of contemporaneous veins. The rock which occurs as veins 

 is necessarily of later date than the rock in which the veins occur, 

 but the difference in point of time is probably not great. Both 

 rocks belong to the same phase of plutonic action. 



Now such cases of heterogeneity in plutonic masses as those 

 referred to may be due to SOKET'S principle ; or to progressive 

 crystallization accompanied by a separation of the first-formed crystals; 



