CONTACT ZONES 



243 



richly charged with mineralizers we may even observe toward the contact 

 such thoroughly reorganized and richly and porphyritically charged schist 

 that this border rock has itself been called a porphyritic granite. 



Contact Zones, especially from Limestones 



Closely akin to the lit-jjar-lit injections and the reorganization of large 

 masses of older wall-rock throughout impressive distances from the igne- 

 ous source are the phenomena distinctively called contact zones. They 

 are portions of the wall-rock adjacent to an intrusive which are so deeply 

 affected by its influence and by its contributions as to be recrystallized 

 often into entirely new minerals. When the wall-rocks are older igneous 

 varieties or quartzose sandstones, but little may be produced in the way 

 of changes. When they are shales or slates, our familiar hornfelses result, 

 often with andalusite, variety chiastolite; often with biotite, sillimanite, 

 staurolite, plagioclase, and occasional garnets. All stages may be traced, 

 from sedimentary beginnings of chiastolites, showing little more than 

 assemblages of carbonaceous pigment, to the beautifully developed, finger- 

 shaped crystals with their included crosses. Since all the chemical com- 

 ponents for the chiastolites and other aluminous silicates are in the shales 

 or slates, there is no occasion to assume necessary contributions from the 

 intrusive rock. But when we come to limestones the case is different. 

 We now deal with some of the most interesting phenomena in the wide 

 range of geology and with the most prolific mineral localities of the world. 

 Over the large part of the areas affected the limestones are, perhaps, 

 changed to crystalline marble, but locally and often for long distances 

 outward they are entirely or in large part recrystallized to lime silicates, 

 of which diopside, wollastonite, garnet^ epidote, vesuvianite, and the 

 scapolite group are most frequent and most abundant. 



Quite naturally the earlier view reached by observers was that the lime 

 silicates only resulted when the original limestone contained silica and 

 alumina in such quantity as to furnish the raw materials for the later 

 formed minerals. Heat, pressure, and the help of hot waters from the 

 intrusive were then deemed sufficient to cause the recombinations. If we 

 start with calcium carbonate, silica as quartz or chert, kaolinite, and, in 

 subordinate degree, ferric hydrate or siderite in the limestone, and end 

 with lime silicates, alumina silicates, and minor iron silicates in the 

 contact rock, carbon dioxide and water have to depart. 



One of the most ingenious and interesting of the early papers of Joseph 

 Barrell is his discussion of the "Physical effects of contact metamor- 

 phism/' 22c wherein, by using percentages by volume instead of by weight, 



