46 THE MODE 01? OCGUKEENCE OF ROCKS 



into which have been extruded and intruded the great variety 

 of igneous rocks which form so conspicuous a feature in many 

 a mountainous region. In order to properly understand that 

 which is to follow, a little space may well be devoted to a 

 consideration of the manner in which these rock masses occur, 

 so far as exposed to investigation. 



Several varieties of igneous rocks, and particularly the gran- 

 itic types, occur in the form of immense oval or rounded masses, 

 protruded into overlying materials which dip away on all sides ; 

 such forms are ordinarily designated as bosses. (PL 1.) It is 

 a form common to granite, gabbros, norites, etc. A laccolith 

 is a somewhat similar form due to the welling up of a magma 

 through a comparatively small vent, but which, instead of com- 

 ing to the surface, spread out laterally into dome-shaped masses 

 between the sheets of the overlying strata. When the intruded 

 matter has been so forced into or between overlying bedded 

 rocks as to appear like more or less regularly defined beds, they 

 are known as sheets or sills. Such, as a rule, may be distinguished 

 from superficial lava flows by their like condition of compact- 

 ness along both upper and lower contacts, surface flows being 

 more or less vesicular along the upper portions, owing to the 

 expansion of their included moisture. The name dike is given 

 to an eruptive mass of varying width included between well- 

 defined walls, and occupying a fissure or fault in previously 

 consolidated rocks. Such are inclined at all angles with the 

 horizon, and are usually of very moderate width, but may ex- 

 tend for miles. The dikes in any one region will frequently 

 be found to belong to one or more well-defined systems, each 

 system occupying fissures essentially parallel with one another. 

 Any one dike may remain comparatively uniform in width for 

 long distances, excepting when split up into smaller dikes. At 

 times, dikes may be traced to the parent mass — a boss or lacco- 

 lith — from which they radiate with more or less regularity. 

 The name volcanic neck or plug is given to the cylindrical mass 

 which results from the congealing of that portion of the lava 

 which remains in the volcanic vent when eruption ceases. 

 Through the erosion of the matter composing the cone of a vol- 

 cano, such are sometimes left exposed owing to their superior 

 hardness, forming very striking featu.res of the landscape. The 

 general name lava is applied to any igneous rock, regardless of 

 geological age or mineral composition, which has been poured 



