H. S WASHINGTON DBCCAB SALTS AM) PLATEAU BASALTS 



extends from X 9 South Carolina, east of the Appalachi 



This may be called the Palisadan region, after -J. D. Dana's old appella- 

 tion, the Palisade Mountains, for the - ies f block ranges belonging 

 '/ -irk ?7=TrH.- 

 In their general characters — physical, petrographieal. and chemical — 

 the igneous rocks of these regions resemble very closely those of the re- 

 gions already described. Hie flows have issued from fissures, with little 

 or no expL: bwc, : ^r-bnilding activity: the sheets are. or originally w 

 horizontal and of great areal extent, and the aggregate thickness of the 



The rocks are predominantly basaltic, either intrusi' diabase 



or surface flows of basalt, with ash beds in small amount. Bhyolitic £ 

 accompany the Algonkian basalts, but these appear to be generally ab- 

 in the Palisadan region. The basalts are composed very largely of augite 

 and labradorite. in more or less equal amounts, with small amounts of 

 magnetite and apatite. The augite would appear to be. a: least in the 

 Palisadan region, an en- . imilar to that of the Decern and 



Thulean regions. A little quartz is frequently pr^ - - me of the 



Palisadan diabases risir g :h quantir >rify the name quartz 



diabase for the rock, and frequently forming (with feldspar) interstitial 

 micTopegmatitic pat: >n the other hand, olivine is not uncommon, 



and this again is sometimes so abundant as to justify the names olivine 

 diabase or olivine basalt. Gl is frequent in the Palisadan basalt, and 

 it is noteworthy that, according to the excellent descriptions of L- 

 the glass content increases, the amount of augite and magnetite decree - - 

 definite magnetite crystals being replaced by "dust*" or minute dendritic 

 growth- ^s we have seen to be the case with the Deccan basalts. In 



megascopic texture these basalts are very uniform and the type with 

 abundant feldspar phenocr; rsts ; "rery rare, in con — ith the other 



plateau be oile phen ysfc >f augite or olivine appear to be more 



common. 43 



Chemically the Algonkian and Palisadan basalts and dial - - greatly 

 resemble those of the Deccan. Oregonian. and Thulean regi od 



analyses of the Algonkian rocks are not numerous, and many of them ap- 

 pear to be of not very fresh specimens. The Palisadan diabases show con- 

 siderable variation, the silica in the anal _ 

 varying from 60.05 to - ~hile the basalts are much more uniform. 



« Cf. J. D. Dana : Manual of Geology, 4th ed_ 1895. p. 35i 



-are on the petrography of the Al-onkian and Palisadan basalts 

 extensive that ft can not be cited here. For the Algonkian rocks reference ma y he 



-uie. and for the Palisadan rocks to J. V. Lewis: GeoL Surr Ann. 



