Handbook of Paleontology 71 



when it is not possible to determine the dark mineral. 

 Trap is the term used for dark dense basalts or diabases. 

 Weathering sometimes gives a greenish cast to old trap 

 or basalt rocks, and they are then referred to as green- 

 stones. 



Structures in igneous rocks. Certain structures are 

 to be looked for in igneous rocks besides changes such 

 as marginal variation in composition which takes place in 

 both the igneous and country rock along the contact zone. 

 Inclusions of the country rock have already been men- 

 tioned; also the development of porphyritic structure and 

 pegmatite dikes. Among other structures to be looked 

 for are vesicular and amygdaloidal structures, miarolitic 

 cavities, jointing and columnar structure. Vesicular 

 structure is common in extrusive sheets and may some- 

 times be found in rocks intruded under pressure. It is 

 due to the escape of water and other vapors from the 

 soft, but stiffening, molten material, rendering it spongy. 

 This happens particularly in the dark basaltic lavas. 

 These cavities, especially the smaller ones, later become 

 filled with various minerals, quartz, calcite etc., and this 

 is termed amygdaloidal structure from the Greek word 

 for almond. The larger cavities are often not entirely 

 filled but minerals project in crystals from the walls. 

 During the process of crystallization there is a contrac- 

 tion of volume in cooling magmas forming minute inter- 

 spaces or pores between the grains, and sometimes dis- 

 tinct cavities are produced, the walls of which become 

 lined with large, well-formed crystals of the minerals 

 composing the rocks. These are known as miarolitic 

 cavities from the Italian name for a granite in which they 

 occur (miarolo). The cooling and concentration of a 



