Notes on the Geology of the Country around Goulburn. 61 



STRUCTURE AND COMPOSITION" OF STRATIFIED ROCKS. 



Having glanced at the relative distribution of the various 

 groups, allusion may now b" made to their structure and com- 

 position, their mode ot arrangement, and also their relations with 

 one another. 



Endless varieties of stone, depending upon differences in com- 

 position, hardness, colour, texture, and structure, are grouped 

 together under the yellow colour, because, in spite of their 

 differences, they possess a common origin, being all sedimentary 

 deposits of a siliceous, argillaceous, or mixed character. They 

 represent an enormous pile of strata, which have experienced 

 great chemical and physical changes since the time when they 

 formed loose mud and sand at the bottom of a sea. And the 

 result of these changes has been to obliterate their original 

 features more or less, and to present them to us nowadays in a 

 metamorphosed state. In hardness they range from flint down 

 to soft material easily dug into. They are white or tinted with 

 all the dull shades which iron can impart. In texture they are 

 fine or coarse, and sometimes conglomeritic. A.nd, by their 

 structure, they can be divided into several varieties — the laminated, 

 having their layers arranged in a horizontal manner, the schistose 

 or foliated, when arranged in wavy plates, and also the cleaved or 

 slaty, when the rock splits readily along planes which do not coin- 

 cide with the lines of bedding. But rocks exhibiting these 

 varieties of structure are indiscriminately mixed together, and 

 shade gradually into one another ; and many rocks can be met 

 with arranged in thin strata or laminations on a large scale, while 

 the subordinate layers are schistose. 



This heterogeneous collection now under review comprise 

 sandstones, quartzites, quartzose conglomerates, hornstones, 

 altered shales, slates, and schists. 



In some places ordinary sandstone is met with, but it is 

 generally too friable to be of any use for building. One of the 

 commonest sorts is argillaceous, has a fine grain, is tinted yellow 

 or red by iron, and crumbles on exposure into small fragments ; 

 the grains of sand and pebbles of most of these ancient rocks 

 are now vitrified, or cemented together, thus forming quartzites 

 and quartzose conglomerates (like specimens shown). The quartz- 

 ites are generally white ; the original grains of sand can easily 

 be distinguished in them ; they are exceedingly hard, and when 

 they occur interstratified with softer material their less rapid 

 rate of waste causes them to project in bold outcrops. The 

 quartzites are generally massive or laminated, but on parts of 

 the Currowang Range, between the Currawang Copper mines 

 and Lake George, the quartzite is schistose (like the specimen 

 shown). 1 have made use of the term hornstone to include a 

 number of rocks which resemble stratified flint or jasper ; they 



