

260 STRUCTURAL AND FIELD GEOLOGY 
all. The accompanying section (see Fig. 105), by Mr J. D. 
Kendall, tells its own tale. Here the replacement of iie 
limestone is rendered conspicuous by the shaly partings and 
layers which traverse the hematite, and are obviously 
continuous with the similar layers and partings in the lime- 
stone at 41. The limestone has been transformed into ore, 
while the argillaceous shales (with which no chemical reaction 
could take place) remain unchanged. ‘There is, moreover, a 
gradual transition from the hematite into the limestone—the 
one is not sharply marked off from the other. It may be 
added that the characteristic fossils of the limestone are often 
partly or completely changed into iron-ore. Similar pheno- 

Fic. 105.—-METASOMATIC REPLACEMENT OF LIMESTONE By HA:MATITE, 
a, boulder-clay ; b, limestone; b!, siliceous; c, c, shales; F, fault; d, hematite replacing lime- 
stone; 0, 0, sides of the open cut. (After J. D. Kendall.) 
mena occur in limestones and dolomites of various ages 
elsewhere. Thus, in Carinthia, Triassic calcareous rocks are 
metasomatically replaced by ores of zinc, while in Nevada, 
Utah, and other regions in North America certain limestones 
have been extensively converted into silver-ores. 
2, Impregnations.—Refererice has already been made to 
the impregnations which so frequently affect the walls of 
certain lodes and the rocks of a Stockwork. In cases of this 
kind the ores occur partly as disseminations (ze. they occupy 
pre-existing pores and interstices), and partly as metasomatic 
replacements. For example, in a granite impregnated with 
tin-ore we frequently find the ore not only occupying minute 
sm 

ee aero ae ee ee 
4? ee? 
i RR ae lt Rains Ea 
= Teh 

