570 



REPORT OF NATIONAL MUSEUM, 1890. 



in most instances the names which have been applied to the Tertiary 

 and post-Tertiary effusives differ from those applied to rocks of the 

 Paleozoic ages, of which they may be otherwise almost exact equiva- 

 lents.* The following table shows the relationship such as exists be- 

 tween the plutonics and the effusives so far as now known : 



Plutonics. 



Effu 



sives. 



Paleovolcanic. , 



Neovolcanic. 











Quartz- -free porphyries 







Nepheline Phonolites. 







Gabbros, Norites, Diabases 



Melaphyrs and Augite porphy- 

 rites. 



Basalts. 







part). 

 Limburgites. 

 Augitites (?). 









do 





Do 



do 





Do 



...do. 









In the collections the effusives are arranged in the order given below : 

 (1) Quartz porphyries, (2) Liparites, (3) Quartz-free porphyries, (1) 

 Trachytes, (5) Phonolites, (6) Porphyrites, (7) Andesites, (8) Melaphyrs 

 and Augite porphyrites, (9) Basalts, (10) Tephrites and Basanites, (11) 

 Picrite porphyrites, (12) Limburgites and Augitites, (13) Leucite Bocks, 

 (14) Nepheline Rocks, and (15) Melilite Bocks. 



1. The quartz porphyries. 



Composition.— The mineral and chemical composition of the quartz 

 porphyries is essentially the same as that of the granites, from which 

 they differ mainly in structure. Their essential constituents are quartz 

 and feldspar, with accessory black mica or hornblende in very small 

 quantities; other accessories present, as a rule, only in microscopic 

 quantities, are magnetite, pyrite, hematite, and epidote. 



Structure. — The prevailing structure is porphyritic. To the unaided 

 eye they present a very dense and compact groundmass of uniform red- 

 dish, brown, black, gray, or yellowish color, through which are scattered 

 clear glassy crystals of quartz alone, or of quartz and feldspar together. 

 The quartz differs from the quartz of granites in that here it was the 

 first mineral to separate out on cooling, and hence has taken on a more 

 perfect crystalline form ; the crystal outlines of the feldspar are also 

 well defined. Under the microscope the groundmass in the typical 

 porphyry is found to consist of a dense felt-like or felsitic irresolvable 



* The Curator lias not felt it incumbent upon himself here to substitute other 

 names for those now commonly accepted by the best authorities. Such a proceeding 

 would only increase the confusion now existing. 



