406 £xpedition to the 



cerned constituting an uninterrupted chain, crowned with 

 sharp conic summits. In one or two instances, tht- smooth 

 surface of the plain extends unbroken to the base of the gra- 

 nitic mountains; more commonly low parallel ridges of sand- 

 stone mark the boundary of the secondary, and the com- 

 mencement of the primitive regions. It has already been re- 

 marked, that in most instances these sandstones are separat- 

 ed from the granite by no mterposed stratum of older secon- 

 dary, or transition rocks. What will perhaps be considered 

 an exception to this remark, is found at the point where the 

 river Platte descends from the mountains. Here, in the first 

 of the primitive ridges, the rock is an aggregate of feldspar 

 and hornblend. Along the eastern side of this ridge, where 

 the feldspar is in the greatest proportion, it is of the flesh co- 

 loured variety, most common throughout the Rocky Moun- 

 tains, having its unmixed masses comparatively large and of 

 a cubic form. Hornblend and quartz, in small proportions, 

 complete the aggregate. This has the aspect and fracture of 

 granite, and should perhaps be considered a granite, in 

 which hornblend takes the place of mica. Proceeding to- 

 wards the west, the proportion of feldspar decreases rapidly, 

 and its particles become smaller, until at length it disappears,., 

 and the rock consists almost exclusively of hornblend. 

 This is of a distinctly columnar structure, and assumes all 

 the characters of primitive trap. It forms an abrupt spur or 

 ridge, crossing from north to south the narrow chasm which 

 contains the bed of the Platte, pushing that river a little out 

 of its direct course. This narrow ridge, falling off perpen- 

 dicularly on both sides, may well be compared to a close line 

 of palisades placed within, and closing the passage which 

 seemed to promise an entrance into the mountains. The 

 drawing taken on the spot by Mr. Seymour, of which an en- 

 graving has been made for our Journal, displays some of the 

 prominent features of this peculiar formation. The external 

 characters of this rock, seem to us to indicate an origin in 

 no respect differing as to time and circumstances from that 

 of the granite.* Small specimens taken from different points, 



* By adhering too scrupulously to the definitions and principles of 

 European authors, and which are not in all instances applicable to this 

 country, much contusion has been introduced into the descriptions 

 given by American geologists. In particular, we think that sienitic and 

 trap formations have been too much multiplied. If any geologist shall 

 hereafter attentively examine that extensive formation of rocks without 

 mica, and containing hornblend, which extends westward from the gra- 



