230 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



witli pliiii-ioelase somewhat less well developed. Between these there is 

 found tlio quartz, with some accessory orthoclase, and microcline as the 

 last products of crystallization. In some cases these two minerals are 

 j)resent in micropeg'inatitic interg-rowths. A textural variation, which the 

 fades mentioned below also undergo, is from a granular to an imperfectly 

 ojihitic texture. In such cases the order of crystallization of the hornblende- 

 mica and plagioclase is reversed, the plagioclase being most automoi'phic in 

 the ophitic varieties. 



The rock resembles the tonalite described by Becke from the Rieser- 

 ferner.^ It also closely resembles some slides of the typical Adamello tonalite 

 with which I have been able to compare it. The chief difference between 

 theni is that the plagioclase and hornblende have a better crystallographic 

 development in the Crystal Falls rocks than in the Adamello tonalite, and 

 that the accessory allanite of the Adamello rock is wanting in the Crystal 

 Falls tonalite, though the normal epidote may represent it. The horn- 

 blende also differs slightly from that of the Adamello rock in that it is not 

 throughout reddish brown. The central portion of some of the crystals 

 shows this color, but the outer portion is a dirty green, even grading into 

 an almost white hornblende. 



The tonalite grades, by diminution of biotite, with corresponding 

 increase of hornblende, into a quartz-diorite, and by diminution or disap- 

 ^jearance of the hornblende and increase of the biotite into a quartz-mica-diorite. 



Hornblende never occurs alone in the rocks, whereas biotite may 

 occur as the only bisilicate constituent. It is a very common thing" to find 

 in the diorites rounded basic segregations consisting chiefly of mica with 

 hornblende subordinate and just a little accessory feldspar. When the 

 orthoclase and quartz diminish, we get the mica-diorites. Orthoclase is 

 always present in all of these dioritic rocks. In certain facies orthoclase 

 and quartz are very abundant and the ^jlagioclase is correspondingly dimin- 

 ished. Such rocks are clearly plagioclase-bearing granites, and represent 

 gradations between the ordinary tonalite and granite, and point to close 

 relationship of this occurrence with the occurrences nearer Crystal Falls 

 already described, in which the granitic facies predominates and the dioritic 

 facies is subordinate. 



' Petrographische studien am Toiialit der Rieaerferner, by F. Becke: Tschermaks mineral. Mitt- 

 heil., Vol. XIII, 1892, pp. 364-379. 



