[\\TION OP R<< 



the same rock is often so extreme that the beginner would 

 not readily believe them to be from the same mass, union 

 he had himself detached them. Basalt, for instance, on a 

 fresh unaltered fracture, is a compact or finely crystalline 

 rock, heavy, and of an imn Mack colour. Hut 

 weathered cliff it may be seen of every hue from bright 

 yellow to sombre brown, and in many places so soft as to 

 be capable of being dug out with a spade. The beginner, 

 therefore, should on no account omit to make himself 

 acquainted both with the unaltered and the altered con- 

 is of rocks. By degrees he will learn to recognise 

 a rock through all its protean disguises of weathering, and 



uish it even at some . 



2. Apparent Structure and Texture. The nature of 

 the component panicles, and the manner in which they arc 

 arranged so as to build up the mass of the rock, constitute 

 important characters. The geologist in the field has of 

 course only vet . means of investigating these char- 



acters, so that when they become doubtful and obscure he 

 may be compelled to defer the solution of his difficulties 

 until he can find opportunity in-tloors of subjecting the 

 rocks to more detailed and careful scrutiny. But with the 

 aid of his pocket-lens he may be able to recognise three 

 types of structure among rocks which may be termed 

 respci \stalline, Compact, and Fragmental He 



can, therefore, in the field provisionally group the rocks 

 which he encounters under one or other of these sub- 

 ns until more detailed investigation has enabled 

 him to determine their characters with greater precision. 1 



It will be understood that this classification is intended only as 

 a guide for the first identification of rocks in rtsjSM. 



