WARREN. — ALKALI-GRANITES AND PORPHYRIES. 211 



The contact relations of the granite and the associated rocks will 

 be discussed later after these have been described. 



The appearance and general characteristics of this granite are 

 probably as well known as those of any rock in the country, owing to 

 its wide use as a building and ornamental stone and to the descriptions 

 already made of it by Dale and the other authors previously mentioned. 

 In fact the present description is undertaken partly for the sake of 

 bringing into one place the descriptions of all the alkaline rocks of the 

 area, and partly for the purpose of adding certain mineralogical and 

 chemical data of interest. No attempt will be made here to describe 

 several minor ^'ariations in the granite due to local alteration. For 

 these the paper by Dale should be consulted, as well as for many 

 interesting details, relating to the rift, grain and joint structure of the 

 granite. 



Pdrograpkic characters — Megascopic. — The normal type is a holo- 

 crystalline, coarse, equigranular rock (4 to 8 mm.) ; prevailingly gray — 

 light to dark gray, bluish or greenish gray; where altered pinkish, 

 reddish or purplish and greenish. The minerals are : — alkali feldspar, 

 colors as above; quartz, clear glassy to dark smoky, less commonly 

 bluish and opalescent; black, lustrous and beautifully cleaval)le 

 hornblende in prominent, irregular spots, usually intergrown, particu- 

 larly about the margins, with light to dark green aegirite; occasional 

 separate grains of dark green aegirite; very rarely small purplish spots 

 of fluorite and small l)rown zircon crystals. Occasionally the horn- 

 blende-aegirite spots which have suffered alteration are replaced by a 

 soft brownish or yellowish clay-like material. The rock is remarkable 

 for the beauty of its polished surfaces, particularly its darker varieties. 



In places where the granite approaches the granite-porphyry, its 

 marginal phase, the rock is not quite so coarse in grain and the equi- 

 granular texture gives place to a distinct but not prominent, gran- 

 ophyric one. (Rattlesnake Hill type.) 



In the medium gray type of granite, which is by far the most 

 abundant type, occur irregular and ill-defined darker streaks and 

 cloud-like masses which grade more or less insensibly into the other. 

 The darkest phase of the granite is known under the name of " black 

 granite-" and is the most highly prized variety from the commercial 

 point of view. As will be pointed out later, these dark jiortions of 

 the granite appear to owe their darker color to a greater abimdance of 

 the minute crystallizations of riebeckite and magnetite in the feld- 

 spars. While this difference may be due in small measure to slight 

 differences in original composition, it is believed to result from a 



