ANDESITIC PEAELITES. 235 



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

 mica-andesites than to the more basic group. A variety of these pearlites 

 exceptionally rich in glass is characterized by the appearance of hyper- 

 sthene. As it is one of the most basic of this group of andesites it will be 

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

 vitreous texture, breaking readily under a hammer-blow, while others are 

 more or less pumiceous, crumbling easily under atmospheric agencies. 



The largest body of hornblende-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 spread out over large areas, but have piled up 

 in irregular rounded hills, the highest reaching an elevation of 500 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 more 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 Mountain in the 

 larger and least altered bodies. Associated with the more crystalline types 

 occur excellent exposures of andesitic glasses and pearlites, products ot 

 more rapid cooling of the same magma under slightly different physical 

 conditions. Other localities of hornblende-mica-andesite with the accom- 

 panying pearlites closely resembling each other in manner 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 Dry Lake. 

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

 blende-mica-andesite occupy positions quite inferior to the glassy varieties, 

 so far as the amount of extravasated lava is concerned, but it is by no 

 means easy, owing to insensible gradations, to draw a sharp line between 

 the crystalline and glassy types. At the first two localities they occur, 

 breaking out at the southern base of the upturned longitudinal ridges 

 of sedimentary strata. In Spring Valley and along tKe Lookout fault, 

 where the lavas penetrate the mountains on the west side of Prospect Ridge, 

 the glassy varieties have poured out in relatively large masses, pearlites 



