112 GEOLOGY OF THE EUREKA DISTRICT. 



characteristic fauna occurs, in which were found Lingda mantmda, Ortliis 

 hamburgensis, 0. testudinaria, Tiplesia calcifera, and PfgeJioparia haguei. To 

 the north of Roundtop the beds are much broken up by volcanic masses, 

 the structure being most diflficuh to make out and the beds impossible to 

 follow, but beyond this again the beds recover their normal position, stick- 

 ing north and south and dipping at a high angle to the east, until the entire 

 series of beds is lost beneath the rhyolite. Along the east slope of Round- 

 top the Eureka quartzite dips generally eastward, an exception being the 

 block lying between Glendale Valley and the ravine coming down from the 

 north slope of Roundtop. Here it has been thrast violently forward toward 

 the south and dips with a high angle to the southwest, in marked contrast 

 to the main body. 



Along the west slope of Hoosac Mountain both the Hamburg shale and 

 the Pogonip limestone again come to the siirface, the latter rising- within 

 200 feet of the top of the mountain, the line between the two limestones 

 being defined as elsewhei'e by the occun-ence, although poorly preseiwed, 

 of a grouping of species characteristic of the border line between the Cam- 

 brian and the Silurian. 



Hoosac Mountain.— This bold mountain mass, situated to the east of the 

 Hambm-g Ridge, attains an elevation several hundred feet higher than any 

 point along the ridge, rising prominently above the immediate country with 

 an altitude of over 8,500 feet above sea level. The broad summit for 

 nearly one-half mile in length maintains approximately the same elevation, 

 a few points hera and there rising slightly above the general level. With 

 the exception of the narrow strip of Pogonip limestone upon the west slope, 

 the Eureka quartzite forms the entire mountain. The mountain falls off 

 gradually to the north and south, but more or less abruptly to the east, 

 where the quartzite, broken down by a series of small parallel faults, 

 presents numerous low walls and cliffs toward the Hoosac fault. The 

 quartzite body, where it is possible to determine any structure, ti-ends inva- 

 riably north and south and dips easterly, but nothing can be made out as to its 

 thickness, owing to the great amoimt of local displacement. The quartzite 

 resembles the horizon as seen elsewhere, except that it is more or less altered 

 by solfataric action and by the intrusive rocks, which penetrate it as narrow 



