28 



NATURE STUDY REVIEW 



[9:1— Jan., 1913 



Figure 3. — Gneiss from Leete Island Quarry. Showing 

 gneissoid texture due to squeezing and stretching of granite. 



rock is sometimes called a porphyry. It is better though to use 

 the term in connection with the proper rock name, thus, granite- 

 porphyry, syenite-porphyry, etc., or basalt-porphyry. A rock is 

 said to be coarse-grained when the crystals are larger than peas 

 and fine-grained when they are smaller than fine shot. It should 

 be noticed that the crystal grains that go to make up the rock are 

 rarely ever perfect in shape. As they grew they crowded against 

 each other and were thus prevented from assuming their true 

 form, so that they appear in the rock as more or less irregular 

 grains. Joints are very common in the massive igneous rocks. 

 They may be at all angles, from vertical to horizontal. These 

 joints are often of great assistance in quarrying the rocks. 



There are a great many varieties of igneous rocks because 

 there is a wide variation in the mineral composition, but for 

 general use and in the field a series of names have come into 

 use that make it possible to bring similar rocks together into 

 groups under a few simple names, as the following table shows. 



