120 



BULLETIN OF THE 



predominating. In one of the enclosed fragments, quartz was seen 

 containing several of the double pyramidal inclusions so common in the 

 quartz of modern rhyolitcs. 



On the Mineral Point Railroad, abo4it three fourths of a mile from Han- 

 cock, the melaphyrs were seen to be interbcdded with a number of con- 

 glomerates. Certain pebbles intended to represent the different varieties 

 "were collected. No. 419 is a very compact, fine-grained, reddish-brown 

 rhyolitic rock (felsite). In the thin section it shows a compact ground- 

 mass, made up of the devitrification products : quartz, feldspar, and ferrite. 

 No. 420 has a reddish-brown groundmass, holding reddish feldspars and 

 black hornblendes. The feldspar is seen in the thin section to be much 

 decomposed, and part holds quartz in the graphic form in it. Some 

 retains traces of its triclinic character, but all has the same hematite 

 alteration' product that is common in orthoclase. The hornblende crys- 

 tals of this old trachyte are changed to ferrite and viridite. Besides the 

 quartz, ferrite, and viridite, as another alteration product cpidote occurs. 

 This mineral is quite abundant. Magnetite and some apatite were seen. 

 No. 421 has a very compact, reddish-brown felsitic groundmass, holding 

 feldspar crystals. This old trachyte (feldspar porphyry, or felsite), in 

 the thin section, is seen to have a reddish-brown groundmass, composed 

 of alteration quartz, feldspar, and ferrite. The porphyritically enclosed 

 feldspars are largely plagioclase. No. 422 is another trachyte (felsite 

 porphyry), containing numerous reddish and greenish-gray feldspar crys- 



tals, held in a dark, reddish-brown groundmass. Microscopically the 



4 



feldspars and groundmass are seen to be greatly decomposed and kaolin- 

 ized. Viridite, epidote, opacite, and quartz occur as alteration products. 



This rock is closely allied to the andesites. 



No. 423 is a very fine-grained reddish granite, composed of feldspar, 

 quartl, hornblende, and biotite. Considerable alteration quartz, fcriitc, 

 and opacite were seen. The feldspars are much altered. The quartz 

 contains fluid, vapor, and stone cavities, also numerous trichites. 



No. 424 is a brick-red granitoid trachyte (granite porphyry). The 

 feldspar is much decomposed, and has the alteration quartz arranged in 

 the graphic form. The rock shows through its alteration a somewhat 

 spherulitic structure. This arises from the radiating arrangement of 

 the alteration products in the groundmass. The sandstone found asso- 

 ciated with the conglomerates is made up of the fine detritus of these 

 trachytic and rhyolitic rocks, mixed to a greater or less extent with the 

 basaltic materiah 



The sandstone and conglomei'ate west of the traps were studied in sev- 



■ ■■j-uvJ— J■-»VJ^^>Lv- rmj— r:ji.^->n ^— -|*^.-Ki 



ymiiii3itfci^»iz>**^.aw_^ofl-'i>r^ia2Kfc:^^^ ■-'lllI*^^|■SL^S1tSZlJ■^ 



