THOUSAND SPEING VALLEY. 519 



minute crystals of quartz. The groundmass shows stripes and bands of 

 darker and lighter material, which encircle in wavy lines these druses, and 

 the sanidins and quartz. Small crystals of free quartz and feldspar are 

 visible throughout the mass. Some portions of the rock abound in com- 

 paratively large lithophysse, cavities which resemble bladder-like swellings 

 in the molten material of the rock, but which are generally regarded by 

 microscopical geologists to resvilt from a chemical and mechanical alteration 

 of large sphserulites. Under the microscope, the stripes in the groundmass 

 are seen to be made up of three characters of bands. The middle is of a 

 grayish-yellow color composed of sphserulites and small fibres. On either 

 side are dull gray zones of fibrous, rather homogeneous material, bounding 

 which are third zones of colorless crystalline aggregations. 



In a little group of hills to the west of the northern point of the Toano 

 Group, which forms the southeastern border of Thousand Spring Valley, 

 are similar shales and limestones, with an easterly dip. Here the develop- 

 ment of limestone is heavily bedded, generally of a grayish-blue color, and 

 seamed with white calcite. A very prominent zone of nearly black siliceous 

 limestone has almost the hardness of quartz, although it effervesces freely 

 with acids, leaving a black porous mass. Under the microscope, it is seen to 

 contain crystals of white calcite and opaque black particles which are 

 doubtless carbonaceous matter, to which the rock owes its dark color. 

 These limestones have been assigned to the Upper Coal-Measure series, 

 rather on account of their shaly character and relation to those of the 

 Toano Group, than from any definite structural or palseontological resem- 

 blance to this series. 



Thousand Spring Valley is a broad, shallow depression, covered with 

 finely comminuted Quaternary material, which takes its name from the num- 

 ber of small springs found along the bottom and adjoining the bed of Pass- 

 age Creek, which is itself dry during portions of the year in the greater 

 part of its course. These springs present a great range of temperature, 

 from the boiling-point of the hot springs at the upper part of Passage 

 Creek, to a temperature of near the freezing-point in little springs which 

 ooze out of the muddy bottom of Passage Creek, just north of the limits of 

 the map. In the higher portions of the valley, along the borders of the 



