Revleir ()f Recent Geologieal Literature. 55 
We luxve often wondered at the vast amount of work accom- 
plished by the director of the 'United States Geological vSurvc}', 
and have admired his ability and consummate tact in the manage- 
ment of the many interests intrusted to him. We do not wish to 
throw a straw in his w'ay, but rather to relieve him of a portion 
of his labor, and at the sam(» time to establish two of the great 
enterprises whicli he htis in hand on sure and recognized bases. 
In that we aim as much to individualize and strengthen the geo- 
logical survey tis to cori-ect and fortify the topographical 
survev. 
REVIEW OF RECENT GEOLOGICAL 
LITER ATIJEE. 
Till \'iilc(nnc /{(icLk <if Siiu.ili Minnitalii in I'ln nsijl ni ni<i <iii<l Mdii/hniil . 
By (tI'.okok II. Wii.i.i.vNfs ; Anier. Jour. Sci., vol. xiiv, j)]). 482-496, Dec, 
1892. The object of this article is to announce the identiiication of a 
large area of volcanic rocks which make up an important ])ortion of 
South mountain. Tliese rocks show abundant evidence of their vol- 
canic origin in their structure, chemical composition and petrogra- 
phy. South mountain has been studied and described in more or less 
detail by Henry Itogers, Tyson, Fi-azer, Hunt and Lesley; but none 
of these geologists seem to have completely recognized the true nat- 
ure and genesis of tliese roci<s. Tliey have liitherto been known as 
felsyte, argillyte, petrosilex, chlorite-slate, epidote-slate, etc., and 
their origin has usually been ascribed to sedimentary agencies. The 
reasons for the latter fact are that the jointing and slaty structure of 
the rocks, althougii of secondary origin, has been taken as ])roof of 
sedimentation, and tluit no one familiar wifli recent voh-iuiics lias ex- 
amined tliem. 
The igneous rocks of South mountain occupy au area of about 175 
square miles and, so fai- as examined, are found to lie of two classes — 
acid and basic, with tlieir associated pyrociast ics. Tlie acid ones are 
usually porphyritic and are found to exhibit the characteristics of 
recent glassy and half-glassy rocks — such as llow-structure, jierlitic 
structure, litlio|)liysa', splicrtditcs, axiolites, etc. — in hardly less per- 
fection than those described by hidings from the Yellowstone jiark. 
A characteristic feature of the rocks under consideration is the 
eutaxic structure, i. e., the intermingling of two portions of the mag- 
ma whicli sliow differences in color and chemical composition ; when 
these mingle in interlacing bands there is some resemblance to sedi- 
