MOSQUITO FAULT. 199 



debris slopes east of the fault line. Its material is a silicioiis rock, more 

 or less iron-stained and of somewhat cherty nature, in some places honey- 

 combed and porous like the quartzite knob adjoining the London fault on 

 Pennsylvania Hill. Its structure lines, though somewhat indistinct, appear 

 to indicate a vertical dip in the stratification, and among the fragments at 

 the upper part of the cliff are some of limestone, resembling the base of the 

 White Limestone. All this material is too much metamorphosed to permit 

 of an absolute identification of its original character, but it evidently belongs 

 neither to the Archean on the east side of the fault, nor to the Upper Coal 

 Measure beds on the west. It is fair to assume, therefore, that it represents 

 a portion of the Cambrian and, possibly, of the Silurian beds belonging to 

 the steep western side of the fold, which once arched over the top of Bartlett 

 Mountain (as indicated by the dotted line in Section A), and which, b}' the 

 friction and pressure that accompanied the displacement of the Mosquito 

 fault, have been compressed and metamorphosed until they are no longer 

 recognizable. Further evidence in favor of this view is found on Little 

 Bartlett Mountain, a continuation of the Bartlett Alountain ridge just beyond 

 the limits of the map, upon which a fragment of Cambrian beds, consisting 

 of characteristic saccharoidal quartzite, is found capping the summit and 

 extending part way down the western slope, with a dip of 45° to the north- 

 west. They end in a little cliff a few hundred feet above the fault line, on 

 which the contact with the underlying granite is well exposed; a bedding 

 parallel to that of the quartzite can be traced for some distance into the 

 granite, with an apparent arrangement of the feldspars in layers parallel to 

 this bedding. The actual contact consists of the usual fine-grained con- 

 glomerate, with small rounded pebbles of limpid quartz. The quartzite at 

 the summit of Little Bartlett Mountain alternates with Lincoln Porphyry in 

 such manner as to make it probable that the latter is a relic of an inter- 

 bedded intrusive sheet. 



Arkansas amphitheater. — In the Arkansas amphitheater, remarkable for its 

 semicircular form and magnificently steep eastern walls, which rise 1,500 to 

 2,500 feet above the stream bed, the debris in its bottom gives evidence of 

 a very lai'ge development of eruptive dikes, among which hornblende-por- 

 phyrites and biotite-porphyrites play an important part. Nearly all the 



