GROUPS OF VOLCANIC ROCKS. 35 



pressure in the earth ; but no clear line of demarcation can bf 

 drawn between these groups, because the conditions of eruption and 

 consolidation are such that every possible gradation of pressure and 

 slowness of cooling must be presented by varying circumstances. 



It is probable that every kind of plutonic rock has its volcanic 



^^^ VOLCANO 

 CLEAVAGE FOLIATION 



BEDDING 



CLAY SLATE GNEISS GRANITE 



Fig. ix. 



representative; but the volcanic form which the plutonic rock assumes 

 has not always been identified. Long since it was observed through- 

 out Europe that the older Tertiary volcanic outbursts were of the 

 kind of volcanic rock which is called trachyte, while the later out- 

 bursts are basalts. And this observation led to an attempt to classify 

 the igneous rocks into two classes, of which these rocks are types, 

 according to certain differences in chemical composition which governed 

 the appearance in them or absence of certain materials. The im- 

 portant variable constituent was found to be silica. In the basalt family 

 this substance usually forms 45 to 55 per cent, of the rock ; while 

 in the trachyte family the silica may be 60 to 80 per cent., or more. 

 This is an artificial classification, but is convenient in obtaining a 

 general idea of the relations of the rocks to each other ; because wher- 

 the quantity of silica is small, quartz does not form in separate crystals, 

 and only those varieties of felspar are produced which contain little 

 silica; and augite or hornblende is more frequently formed than 

 mica. This division was made by Bunsen, who named the group 

 which contains but little silica, Basic, and that which contains much 

 silica, Acidic. The association of minerals together is almost always a 

 consequence of the original chemical composition of the mass out of 

 which they were formed ; thus, Cotta remarks, quartz and mica occur 

 together ; orthoclase quartz and mica ; orthoclase or oligoclase and 

 hornblende ; labradorite and augite ; but quartz and augite rarely, if 

 ever, are associated. Many of the volcanic rocks appear to owe their 

 varieties to the influence of heated water penetrating to great depths, 

 which has dissolved the silica and certain other minerals met with in 

 its course, and added these to the heated mass, which was afterwards 

 poured out at the surface. And since such large quantities of salts 

 ire dissolved in the waters of mineral springs, it is impossible to 

 >ver-estimate the changes which may be produced by this means in 

 leep-seated heated rocks, and it may sometimes be the true explanation 

 )f the circumstance that when the two classes of lavas occur together, 

 older series is often the richer in silica. We now propose to give 

 short account of some of the chief varieties of the principal groups 

 )f igneous rocks, enumerating a few localities in which they occur 

 ind may be studied. All igneous rocks are divided first into those 

 rhich con tain orthoclase, and secondly, those formed by plagioclase ; each 

 )f these groups includes two families, quartz-bearing and quartz-free. 



