THE EARTH'S ROCK FOUNDATIONS 69 



of them are relatively fine crystals, the rock is again known as a 

 porphyry; and in this case porphyritic diorite or diorite porphyry. 



In the gabbro-basalt group, the rocks consist essentially of 

 pyroxene and feldspar, and the feldspar is usually of the dark 

 variety, labradorite being the commonest form, though we do 

 have gabbros in which orthoclase is abundant. The pyroxene in 

 these rocks equals or exceeds the amount of feldspar present. 

 These rocks are all dark in color, relatively heavy, and the amount 

 of quartz present is small. Very often there are accessory 

 minerals present such as mica in tiny flakes, particles of hematite 

 or magnetite, and often olivine in considerable quantity. In the 

 latter case the olivine gives the rock a distinctly greenish cast and 

 such rocks are commonly known as green stones. Gabbro is the 

 coarsely crystalline member of this group. Diabase is more 

 finely crystalline. If the crystals of which the rock is composed 

 are quite fine, the feldspar being recognizable but the accompany- 

 ing darker minerals in such fine particles that it is difficult to 

 distinguish them even with a lens, the rock is a dolerite. 



The term, basalt, is used to include all of those dense, dark, 

 igneous rocks in which the constituent grains are so tiny as to be 

 unrecognizable. Sometimes one of the ingredient minerals will 

 be present in coarse crystals, when the rock is known as a basalt 

 porphyry. Basalt occurs in very large beds, covering immense 

 areas, especially in the regions occupied by the older rocks. As 

 the old lava cooled, giving rise to the basalt, often the mass so 

 contracted as to break into quite regular columns, and these 

 shattered into blocks by cross-fractures so that not uncommonly 

 basalt has a columnar structure. A similar phenomenon is 

 seen in beds of mud where the clay on drying cracks into polyg- 

 onal masses. In the latter case the phenomenon is due to loss 

 of water, whereas in the former it is due to the gradual contrac- 

 tion as the hot mass cools. Such columnar basaltic masses are 

 famous in the Giant's Causeway in Ireland, the Devil's Pile 

 Quarry in our western states (see also Fig. 29). Basaltic tuff is 

 very light, spongy rock, dark in color, and correspondingly 



