376 /. MORGAN CLEMENTS 



and shows its usual characters. It is less well developed than 

 the hornblende. All of the other minerals show their usual 

 well-known characters. 



According to the variations in the proportions of the essen- 

 tial minerals, plagioclase, orthoclase, quartz, hornblende, and 

 biotite, we get the following varieties of diorite : quartz-diorite, 

 tonalite, quartz-mica-diorite, and mica-diorite. This variation 

 is best shown in a connected group of knobs composed of 

 medium grained rock, which varies in color from light pink to a 

 very dark greenish-gray. Areas of light and dark colored rock 

 may be seen extending in finger-like projections into one 

 another. There are no sharp boundaries between them, how- 

 ever, but, on the contrary, a gradual passage from the light to 

 the dark variety. Some of the rocks also contain small, more 

 or less rounded areas, which are clearly segregations of ferro- 

 magnesian minerals. These different phases evidently belong 

 to a single rock mass, but the microscope enables a separation 

 to be made into the several mineralogical and textural facies 

 already mentioned. The main mass of the rock forming the 

 knobs appears to be a tonalite which resembles the published 

 description by Becke 1 of that from the Rieserferner. It also 

 closely resembles some slides of the typical Adamello tonalite 

 with which I have been able to compare it. This tonalite grades 

 by diminution of biotite with corresponding increase of horn- 

 blende into quartz-diorite, and by a diminution or disappear- 

 ance of the hornblende and increase of the biotite into a quartz- 

 mica-diorite. Where the quartz of this last variety is practically 

 wanting, we get a mica-diorite. In this small massif pure types 

 of the rocks described are of rare occurrence. Orthoclase is 

 present in all of these dioritic rocks. In certain facies the 

 orthoclase and quartz are very abundant, and the plagioclase 

 correspondingly subordinate. Such rocks approach closely the 

 plagioclase-bearing granites. 



The relationship to granites is better shown in a different 



1 Petrographische Studien am Tonalit der Rieserferner, by F. Becke, Tsch. Mitt. 

 Vol. XIII, 1892, pp. 379-464. 



