40B Expedition to the 



cent secondary aggregates with which they are associated. 

 In other instances, black and formless masses of porous and 

 amygdaloidal substances are seen scattered about the plains, 

 or heaped in conic piles, but having no immediate con- 

 nexion with the rocks on which they rest. An extensive 

 formation of the rocks belongmg to this class occurs along 

 the eastern slope of the Rocky Mountains, about the sources 

 of the Canadian, and the Red river of Natchitoches. Among 

 them we distinguish two leading varieties referrible to the 

 tAvodivisons called Greenstone and Amygdaloid. 



1st. — Greenstone. It appears in the limited district we 

 examined, under almost every variety of form and character 

 noticed by mineralogists. Sometimes it is nearly or quite 

 free from any intermixture of hornblend, and is of a fine 

 dark green colour, and closely resembles some varieties ot 

 serpentine. Sometimes its colour is a dull gray, graduating 

 into brown and black of various shades and intensities. It 

 forms numerous conic hills of considerable elevation, scat- 

 tered without order in various parts of the plain. These hills 

 are usually of a regular and beautiful form. The great plain 

 in which they stand is elevated and destitute of timber or 

 water, but ornamented with a carpet of thick and verdant 

 grasses. The hills, though steep and high, are smooth and 

 green to their summits; the surface on all sides being unbro- 

 ken by trees or rocks, and covered with thick turf; the whole 

 forms a scene of singular beauty. During our journey across 

 the district based upon the rocks now under consideration, 

 we had constant occasion to admire the freshness and abun- 

 dance of the grasses and otner herbaceous plants. The 

 plains of the Platte and Arkansa we had seen, brown and de- 

 solate as if recently ravaged by fire, but here we passed ele- 

 vated tracts where for many miles we could find no water 

 for our necessities, yet the vegetation possessed the fresh- 

 ness of spring in the most fertile regions. The greenstone 

 sometimes appears in low ridges, extending considerable dis- 

 tance, and sloping on both sides into the level of the plain. 

 In the sides of ravines and water-courses, perpendicular 

 precipices of considerable elevation occur, but the vallies 

 are usually almost filled with large broken masses, and 

 fragments sometimes exhibiting a prismatic form. It falls 

 rtitdily into large masses, but seems strongly to resist that 

 pngress of di?iiUegration, which it must undergo before it 

 can he removed by iht- water. The faces of the perpendicular 

 precipices are often marked by distinct and large seams run- 



