HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 517 



or geological age, but upon the conditions under which it cooled from 

 a molten magma, portions of the same rock varying all the way from 

 noncrystalline granular through porphyritic to glassy forms. To this 

 fact allusion has already been made. 



The thin sections from which these transparencies were prepared are 

 shown in the case. The actual portions of the rock shown in each 

 transparency is that surrounded by the dark ring in the section. 



The transparencies are as follows : 



(1) Crystalline Limestone, or marble, from West Rutland, Ver- 



mont. Transparency No. 39074. 

 The transparency shows the stone to be made up wholly of calcite 

 crystals. Observe that, owing to their crowded condition, none of the 

 crystals have perfect crystallographic outlines, but have mutually in- 

 terfered with one another's growth, giving rise to rounded and angular 

 granules only. The striations, cutting at various angles across the 

 granules, are cleavage lines and twin laininellre. (See also Fig. 93, 

 p. 545.) 



(2) Granite, Sullivan, Maine. Transparency No. 39075. Composi- 



tion, quartz, feldspars, and mica. 



The clear colorless and the brilliant blue portions are quartz; the 

 clouded portions sometimes bounded by longitudinal parallel striations 

 are the feldspars, while the faintly yellowish almost opaque forms in 

 small shreds near the center are black mica. 



Observe that in this case as in that of the limestone the rock is fully 

 crystalline, but that none of the minerals possess perfect crystalline 

 outlines, owing to a mutual interference during the process of their 

 formation. Such a structure is called crystalline granular, or more 

 technically hypidiomorphic. It is a structure characteristic of plutonic 

 or deep-seated rocks. (See Fig. L, PI. cxx.) 



(3) Diabase, Weehawken, New Jersey. Transparency No. 39076. 

 This rock is composed mainly of the mineral augite and a triclinic 



variety of feldspar. The clear, colorless elongated forms often showing 

 a parallel banding are the feldspars, and the large irregular forms of a 

 bronze yellow and green color are augite. 



Observe here that while the rock, as in the case of the granite, is 

 wholly crystalline, the various minerals have interfered less in process 

 of growth, giving in part very perfect crystalline forms. Such a struc- 

 ture is technically called panidiomorphic. (See also Fig. 9G, p. 562.) 



(4) Serpentine, Chester, Pennsylvania. Transparency No. 39077. 

 Serpentine is not known in crystals, but occurs as an amorphous 



product of alteration after other minerals. The section shows the 

 characteristic reticulated structure. The interspaces in this case were 

 filled with calcite and frequent grains of chrome iron. (See also Fig. 6, 

 PL cxx.) 



