AKDESITIC PEARLITES. 235 



scopic mineral, and relates the pearlites more closely to the honiblende- 

 mica-audesites than to the more basic gi'oup. A variety of these pearlites 

 exceptionally rich in glass is characterized by the ap})earance of hypci-- 

 sthene. As it is <me of the most basic of this g-rouj) of andcsitcs it will l)e 

 discussed farther on in this chapter. Many of the varieties possess a dense 

 vitreous textiu'c, breaking readily under a hauimer-blow, while others are 

 more or less pumicenus, cruiuljling easily under atmospheric agencies. 



The largest liody of hornbleude-andesite occurs as a fissure eruption 

 along the Hoosac fault, the lavas coming to the surface just to the south of 

 the junction of the Ruby Hill branch with the main fault and extending 

 southward till lost beneath rhyolitic flows. Like most acidic rocks these 

 extravasated lavas have not s})read out over large areas, but have ])iled up 

 in irregular rounded hills, the highest reaching an elevation of oOO feet 

 above the base of the limestones along the fault line in the valley. For 

 the greater part of the distance along this fissure these lavas have under- 

 gone Uiore or less alteration, due to solfataric action, kaolinization taking- 

 place with the formation of secondary minerals. Comparatively fresh rocks 

 not far from the fault are still found northeast of Hoosac M(nintain in the 

 larger and least altered bodies. Associated with the more cr^•stalline tvpes 

 occur excellent exposures of andesitic glasses and pearlites, products ot 

 more rapid cooling of the same magma under slightly different jjliysical 

 conditions. Other localities of hornblende-niica-andesiti' with the accom- 

 panying pearlites closelj' resembling each other in maimer of occurrence 

 and mineral composition are found at the southern end of Carbon Ridge, 

 in the neighborhood of South Hill, at Spring Valley, and near l)r\- Lake. 

 In all of the four latter localities hornbleude-andesite and the horn- 

 blende-mica-andesite occupy positions (piite inferior to the glassv vari(fties, 

 so far as the amount of extravasated lava is eoncerned, but it is l)\- no 

 means easy, owing to insensible gradatiims, to draw a shar}) lii\e between 

 the crystalline and glass}- types. At the first two localities they occur, 

 breaking out at the southern base of the upturned longitudinal ridges 

 of sedinuMitarv strata. In Spring- Valley antl along tht; Lookout fault, 

 where the lavas penetrate the mountainson the west side of Prospect Ridge, 

 the glassy varieties have pom-ed ont in i-clatively large masses, jjearlites 



