i9o6.] SMITH— PARAGENESIS OF MINERALS. 223 



has caused a partial recrystallization, the plagioclases having given 

 up some soda, which has joined the amphibole molecule, forming 

 secondary needles of a blue glaucophane-like mineral, probably 

 crocidolite. Soda syenite porphyry dykes are said by Mr. H. W. 

 Turner^ to show a secondary blue amphibole where some metamorph- 

 ism has taken place, and he thinks that the albite glaucophane schists 

 were probably made out of such material. 



III. Doubtful Original Sediments. 

 Under this head must be classed many of the basic glaucophane 

 rocks that are interbanded with more acid rocks of sedimentary 

 origin. They were probably originally tuffs, but in many cases they 

 may have been flows or sills. These appear as schists and green- 

 stones, but their field relations often give no indication as to their 

 origin, and they are so thoroughly recrystallized that their original 

 fragmental nature is lost. 



IV. Dotibtftd Original Igneous Rocks. 

 In the Coast Ranges there are many masses of holocrystalline 

 rocks that resemble igneous rocks in their structural relations, but 

 are made up entirely of secondary minerals. Where they, do not 

 grade over into undoubted igneous rocks their origin must remain in 

 doubt, for a chemical analysis can not show whether they were 

 originally fused magmas or fragmental tuffs. These rocks are 

 eclogites, greenstones and basic schists, grading over into each other, 

 and with the chemical constitution of gabbros or diabases, more 

 rarely of diorites. Some of the more basic eclogites may even have 

 been made out of pyroxenite. The eclogites are massive or gneissic, 

 composed of garnet, omphacite, some amphibole, and epidote. The 

 greenstones are essentially omphacite actinolite epidote rocks, and 

 the schists associated with them are usually epidote glaucophane 

 schists with either paragonite or margarite. No eclogites have been 

 found that grade over into dioritic rocks, or have the chemical 

 composition of that group, but the lawsonite schists and gneisses 

 with their high percentage of lime and comparatively low iron and 

 magnesia suggest a derivation from diorites. 



^ 17th An. Rept. U. S. Geol. Survey, Part I., p. 727. 



