LITHOLOGY. IQI 



spotted black; and chlorite granites arc spotted green. Not only is it 

 difficult to generalize on the distribution of these granites, but more than 

 one of them may occur associated together. 



Some features which the granites possess in common may be men- 

 tioned before describing the peculiarities of our individual species, be- 

 cause they indicate the mode of formation of these important rocks. 

 The relationship of the component minerals to each other as regards the 

 order of their crystallization is the first and most evident feature to con- 

 sider. When rocks which without doubt have solidified from a state of 

 igneous fusion are studied, it is found that the least fusible constituents 

 were the first to crystallize. They appear in well defined forms or per- 

 fect crystals, and the least fusible constituents fill the interspaces. But 

 this is all changed when granitic rocks are observed. Quartz is com- 

 monly called infusible, and feldspar is easily fused, yet the feldspar was 

 the first mineral to crystallize, and has obtained its well defined form 

 apparently uninfluenced by the other minerals, while the infusible quartz 

 fills up the interspaces, and was the last to solidify. Although micro- 

 scopic sections are not at all necessary to observe this point, yet in 

 them this relationship, which is so easily recognized macroscopically, 

 is found to extend to the minutest structures. This is illustrated in 

 Fig. 5 on PI. lo, which is drawn from a section of a granite from Cole- 

 brook. The well defined orthoclase crystals are there shown, and the 

 quartz fills the residuum of space. This fact was observed as long ago as 

 1822, when Breislak, in his treatise on the structure of the globe, argued 

 against the igneous nature of granites on the basis of exactly these ob- 

 servations, yet it is only recently that these arguments have been re- 

 garded as conclusive. But observations in the field prove plainly that 

 some at least of the granites have been in a fused or plastic condition. 

 These circumstances must point, therefore, to a plasticity which was 

 induced in some other way than by pure igneous fusion. The relation- 

 ship between the crystallization of minerals and their fusibility extends 

 still further. Granite often contains a variety of minerals, such as tour- 

 maline, garnet, zircon, iolite, fluor spar, etc.; and although all degrees of 

 fusibility are represented, no generalizations can be made upon the order 

 of their crystallization. Such observations as these were the starting- 

 points from which Scrope, Scheerer, and others, supported by Sorby and 



