110 The Sedimentary, Metamorphic, 



imagine to have been originally that of the Silurian sedi- 

 ments when lying still, undisturbed, in a horizontal position ; 

 but it is also far removed from the condition of those forma- 

 tions which have been subject to regional or contact 

 metamorphism. 



Broadly speaking, so far as my investigations have 

 yet gone, the Silurian sedimentary rocks of Gippsland 

 may be arranged under three types. The first is that 

 of the Argillites, or those beds which are found where there 

 are no signs of intrusive masses of plutonic rocks, and which, 

 therefore, are least altered.' The changes which I have 

 observed are usualry some degree of induration by silica, and 

 the conversion of the argillaceous material into some mineral 

 allied to chlorite. The second type is that represented by 

 the well-known rock Hornfels, and includes the contact 

 schists. In such rocks the argillaceous material has been 

 converted into mica, which most frequently is a brown 

 magnesia-iron mica with a subordinate potassa-mica. These 

 rocks are far more indurated by silica than the argillites, 

 and the original clastic grains of quartz are frequently sur- 

 rounded by secondary silica, oriented in accordance with the 

 older grains. The third type, which departs most in mineral 

 character from the normal argillites, includes the so-called 

 Regional Schists. In this group, the first sign of alteration is 

 the minute wrinkling of the argillites, and the appearance of 

 a silky micaceous lustre on the planes of bedding or of 

 cleavage. Silica is also eliminated in the conversion of the 

 argillaceous material into mica, and becomes deposited in 

 strings or lenticular masses in or across the beds. The 

 ultimate result of this type of metamorphic alteration is mica 

 schist and gneiss. 



The distinction between the argillites and the metamor- 

 phic schists, contact or regional, is that in the former the 

 argillaceous material is converted into some mineral allied to 

 chlorite, while in the latter it has been converted into 

 mica. 



The distinction between the contact and regional schists 

 is the more foliated structure of the latter, and the prevalence 

 in them of an alkali-mica. 



Strictly speaking, all the schistose rocks which I have 

 spoken of in this paper should be considered as metamorphic, 

 but I have found it more convenient to separate the beds on 

 the western side of the Tambo River, and to treat them as 

 being sedimentary. In outward general appearance they are 



