418 lixpedition to the 



intimately connected with the rocks of that series, as scarce 

 to require separate consideration. 



VI. — Argillite. 



Of the older secondary rocks, we have observed in the 

 Ozark mountains, only the inclined sandstones and conglo- 

 merates above mentioned, and a limited formation of argil- 

 lite, extending a few miles around the Hot springs of Wash- 

 ita, and re-appearing on the Arkansa, at and above the town 

 of Little Rock, being usually accompanied by vast beds of 

 Petrosilex. This latter ought perhaps to be considered a 

 distinct stratum — but south of the Arkansa, we have not 

 been able to trace it, uninterrupted for any great distance. 



Mr. Nuttall, in his valuable Journal of Travels into the 

 Arkansa Territory, mentions grayxvacke slate as occurring 

 along the Arkans . river, near Little Rock, (p. 105.) Wc 

 have observed none there, in any considerable degree similar 

 to the graywacke slate of the transition mountains of New 

 York, or even to that of the Alleghanies. We are aware, 

 however, that some of the aggregates which we call sand- 

 stones, have all the characters attributed to graywacke slates. 

 " Grayxvacke is a complete sandstone,^''* and, in a district 

 where both are so intimately blended as in that we are con- 

 sidering, perhaps it is unnecessary to attempt any distinction 

 between them; or we may persevere in the use of the two 

 names, at the same time acknowledging they are both ap- 

 plied to the same stratum, f 



The Hot Springs of the Washita issue from clay slate, and 

 if we may judge from the inclination of the laminee, and the 

 distance at the surface from the granite of the Cove, we may 

 conclude that a very large mass of clay slate is interposed be- 

 tween the surface of the granite and the point at which the 

 springs rise. This, however, it is not possible to ascertain. 

 The hottest springs on the globe, rise from beneath or with- 

 in the granite.,:): and it is not improbable this rock may ap- 

 proach near the surface, at many points in the Ozark moun- 

 tains, where it has not yet been uncovered. 



The clay slate about the Hot Springs is highly inclined, 

 often a good deal flinty in its composition; and, as far as we 



* Jamiesou in the Edinburgh Encyclopedia, Art. Mineralogy. 



I Humboldt considers transition schist as the basis of the graywacke in 

 Spain. See Pers. Nar. vol. i, p. 47, American ed. 



I Humboldt's Personal Narrative, vol. iv- p. 171, 195; vol. v, p. 553- 



