14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 66 



to the fissures with secondary silicates. Tliese are principally diop- 

 side and andradite garnet, with less vesuvianite and serpentine, and 

 a little wollastonite, xonotlite, and thaumasite. Magnetite was later 

 introduced in considerable quantity. The order of formation of the 

 most important minerals was diopside, vesuvianite, garnet, serpen- 

 tine, and magnetite. 



This assemblage of secondary silicates is entirely like that found 

 in so-called lime-silicate contact zones and, because of such lime- 

 silicate zones frequently being associated with workable deposits of 

 copper or iron ore, they have been carefully studied by a number of 

 able geologists, and the literature relating to them is rather volumi- 

 nous. A majority of the authorities who have done detailed work 

 on deposits of this type agree in assigning the source of most of the 

 material of the so-called " garnet-zones " to emanations from the 

 cooling magma and consider that there has been a large addition of 

 'material, notably silica and iron, from the igneous rock. There is an 

 alternate opinion supported by some, however, which holds that there 

 has been relatively little material added from the magma and that 

 the lime silicates have formed by simple combination of the lime of 

 the limestone with the impurities already present to form the sili- 

 cates, under the influence of the heat of the intrusive, the excess of 

 calcium carbonate having been removed from the vicinity. It is not 

 desired to enter here into an exhaustive review or discussion of the 

 two theories nor of the various phenomena which characterize lime- 

 silicate zones in general. The literature of the subject has been 

 reviewed in detail in a paper by Uglow '^ who favored the idea that 

 the recrystallization of the materials of the limestone was the process 

 of fundamental importance in the production of the lime-silicate 

 zones. The discussion provoked by this opposition to the favored 

 view was entered into by a large proportion of the leading American 

 authorities on the subject.* The concensus of opinion is that both 

 processes are operative, but the majority favor the conclusion that 

 the addition of material from the magma has greatly overshadowed 

 the mere concentration of impurities by reduction in volume in most 

 of the known cases. 



While the term " contact zones " is generally used for these lime- 

 silicate masses, they are characterized, usually, by marked irregu- 

 larity of distribution, even about a single intrusive mass. In some 

 places great quantities of the silicates are developed at one point 



'W. L. Uglow. Review of the existing hypotheses on the origin of the secondary sili- 

 cate zones at contacts of intrusives with limestones. Econ. Geology, vol. 8, pp. 19-30 and 

 215-234, 1913. 



8 Econ. Geol., vol. 8, 1913, pp. 501-507 (C. A. Stewart), and pp. 597-610 (J. F. Kemp) ; 

 vol. 9, 1914, pp. 73-77 (D. F. Higgins) ; pp. 175-183 (W. L. Uglow) ; pp. 278-281 (C. A, 

 Stewart) ; p. 282 (J. F. Kemp) ; 283-292 (W. Lindgren) ; 292-299 (C. K. Leith) ; 593-594 

 (J. B. Umpleby). 



