66 New York State Museum 



Knob, north of Schuylerville, N. Y. Volcanic necks vary 

 from a few feet to a mile in diameter. In a volcano quiet 

 flows may alternate with explosive eruptions, and a vol- 

 canic cone may be built up entirely of lava, entirely of 

 ashes or cinders or both. Due to expansion of vapors, 

 chiefly steam, eruptive volcanic material has a more or 

 less pronounced vesicular structure. In an explosive 

 eruption the material projected varies from the finest 

 dust to pieces weighing several hundred pounds. De- 

 posits of the finer material form volcanic tuffs; of the 

 coarser material, volcanic breccias. 



The texture of igneous rocks depends upon the rate of 

 cooling, the kind of magma, the place of cooling and also 

 the bulk of the magma. The more slowly a magma cools 

 the more opportunity there is for growth of the mineral 

 crystals, and we have everything from a comparatively 

 slowly cooled rock that is a mass of crystals (holocrys- 

 talline), as granite, to rapidly cooled rocks in which there 

 are no crystals (glassy), as obsidian. A rock that is 

 crystalline but in which the grains are too fine to be 

 visible to the eye is dense. The texture of a rock may be 

 uniform, that is, the crystals of each mineral are of about 

 uniform size. Such rocks are termed granitic or granu- 

 lar and may be coarse-grained or fine-grained. Some- 

 times there are large crystals scattered through a fine- 

 grained or glassy ground mass. Such a rock is porphy- 

 ritic. Rocks according to their composition are acidic 

 or basic. Basic magmas are more fluid and tend to form 

 coarse crystals, therefore producing coarser-grained 

 rocks. Finer textures tend to develop more frequently 

 from acidic magmas. Glassy textures are common here 

 and less frequently developed from basic magmas. The 

 power of forming crystals is increased by the presence 



