300 SCIENCE PROGRESS. 



of a hornblende rock (amphibolite) into a rock having the 

 mineralogical constitution of a quartz-mica-diorite. Traced 

 from east to west, the rock undergoes a remarkable change, 

 quartz and orthoclase as well as plagioclase-felspar being- 

 developed, while simultaneously the hornblende becomes 

 actinolitic, and gives rise to much black mica. This change 

 is ascribed to numerous veins of quartz and potash-felspar, 

 which invade the rock on its western side. 



The question, to what extent an actual addition of 

 material may take place from an intruded igneous magma 

 to the adjacent rocks metamorphosed by it, is one that has 

 been much debated. Although the actual fact of such 

 transference seems to be established in certain particular 

 cases, the conditions which admit of it, and which limit its 

 operation, are still very obscure. The commonest case is 

 that in which the metamorphosed rocks near the junction 

 have received an accession of certain volatile constituents, 

 particularly boric and hydrofluoric acids. The characteristic 

 minerals produced are tourmaline, topaz, axinite, fluorite, 

 certain varieties of white mica, etc., and these minerals may 

 occur in the marginal part of the intrusive rock as well as 

 in the metamorphosed products. Such phenomena are well 

 known in many districts, and seem to be specially connected 

 with intrusions of certain thoroughly acid rocks : the Cornish 

 granites afford good examples. 



The difficulty arises, however, in connection with sub- 

 stances supposed to have been introduced in solution from 

 an intruded magma and to have impregnated the adjacent 

 rocks. The "contact" rocks known as adinoles in the 

 Harz and other districts apparently give evidence of a 

 notable addition of soda and silica. The rocks bordering 

 Monzoni augite-syenites also show an addition of material, 

 and other cases might be cited. On the other hand, the 

 mica-schists described by Williams at Peekskill on the 

 Hudson River seem to show a falling off of silica and 

 alkalies as the junction is approached (23). An apparent 

 falling off in certain chemical constituents may, of course, 

 conceivably be caused by an increased proportion of other 

 constituents of the rock. We naturally look, however, for 



