ERUPTIVE AND RIETAMORPHIC ROCKS. 563 



Through the kindness of Mr. Frank Rutley I am enabled to 

 give the following condensed account, which he has prepared, 

 of the Eruptive and Metamorphic Rocks (pp. 563-568): — 



MetamorpMc Rocks. — Sedimentary deposits which have under- 

 gone any very marked physical or chemical change are usually 

 spoken of as ' Metamorphic Rocks,' but it is often difficult to 

 define their precise claim to this name, since some, in which the 

 alteration is but an incipient one, would not be generally termed 

 metamorphic, while others, in which the same processes have 

 produced a more strongly developed change, would, by many 

 Geologists, be regarded as fit recipients of the title. As a rule, 

 rocks which have undergone any more or less definite alteration 

 from the contact or proximity of heated masses of eruptive rock 

 are considered to be metamorphosed. But there is also very ex- 

 tensive metamorphism to be met with in rocks far removed from 

 any intrusive masses, the changes being partly of a chemical and 

 partly of a mechanical nature, the latter resulting from the move- 

 ments of rock-masses. The crystalline schists have chiefly been 

 produced by such regional metamorphism. 



Chlorite Schist, Mica Schist, Talcose Schist, Schorl Schist, Hornblende Schist, 

 Gneiss, etc., etc., are generally regarded as Metamorphic rocks. Most of these 

 schists or slates contain quartz or sandy matter which usually forms fine folia 

 separating the other mineral components, so that the rock is frequently made up 

 of thin but more or less distinct layers which differ in mineralogical composition. 

 This differentiation into layers of dissimilar character is termed foliation. Foliation 

 may occur in eru]3tive rocks as well as in those which are regarded as metamorphic, 

 and planes of foliation do not, therefore, always represent planes of bedding. 

 Foliation, indeed, has been frequently found to coincide with cleavage. 



Eruptive or Igneous Eocks. — These rocks have resulted from 

 the solidification of molten matter which has been forced upwards 

 from great depths in the interior of the earth, and has either risen 

 through pre-existing fissures or faults, in which case it forms dykes, 

 or it has been intercalated between beds of sedimentary rock, or 

 has formed large irregularly shaped bosses. Again, it may have 

 reached the earth's surface, and have passed out of a volcanic vent 

 either in the form of a lava flow or the matter may have been 

 ejected as ashes or lapilli, which have been deposited perhaps on a 

 land surface, or, falling upon the water, have subsided as sediment. 

 In either case they may exhibit stratification. Molten matter may 

 also be erupted beneath the sea. The Igneous or Eruptive rocks 

 are separated by some petrologists into two groups, which (at the 

 suggestion of Bunsen) are respectively termed 'Basic' and 'Acid.' 

 The former group embraces those rocks which contain under 

 60 per cent, of Silica ; those in which the percentage of Silica is 

 greater being classed in the Acid Group. This classification is 

 of course a purely arbitrary one, and some writers recognize an 

 intermediate group of rocks in which the Silica ranges from 55 

 to 66 per cent. This group includes the phonolites and some 

 trachytes and andesites. 



The minerals that usually enter into the composition of rocks which contain 

 over 60 per cent, of Silica are the following :— Quartz, Orthoclastic Felspar, and 

 sometimes Plagioclastic Felspars, such as Microcline, Oligoclase, and Albite. 



