426 W. H. TWENHOFEL THE SILVER CITY QUARTZITES 



The notes which follow were kindly prej)ared for the writer by Prof, 

 A. X. WinchelL Specimen number 4 came from the brecciated equiva- 

 lent of the latan limestone of locality E, while number 7 was derived 

 from an exposure of the Le Hoy shales near the point D. 



"Number 4. In hand sample the rock is composed of angular fragments of 

 dark green color which have been altered in part (by surface waters?) to a 

 white mineral. These fragments are held together by a heterogeneous cement 

 which includes some material probably detrital in origin and some which has 

 crystallized from solution. The latter is chiefly acicular dark green horn- 

 blende in rosettes. 



"In thin-section the dark green color of the fragments is found to be due to 

 abundant well crystallized lamellfe and needles of dark green chlorite, while 

 the white mineral is chiefly opal or chert. There is also some rare hornblende 

 and rather common grains of epidote which form granular aggregates in some 

 places. The epidote is not only pleochroic, but also varies in color in different 

 crystals from golden yellow to brown and brownish gray. It is biaxial, has 

 high relief and extreme birefringence, indicating an epidote rich in iron. 



"The probable history of the rock is as follows: A siliceous limestone was 

 broken by local movements into abundant fragments of all sizes. The broken 

 zone furnished a ready channel of flow for carbonated waters which dissolved 

 and removed the iron and carbonates from the smaller fragments and from 

 the outer zone of the larger fragments, thus increasing the porosity of the 

 mass. At a later date the same channels served as paths of escape for hot 

 solutions coming from greater deptlis, and these removed any remaining car- 

 bonate and at the same time caused the growth of chlorite, epidote. and horn- 

 blende. Such solutions were probably wholly or partly of igneous origin. 



"Numl)er 7. In hand sample the rock has a light grayish green color with 

 very minute brightly reflecting white scales. It weathers to a darker brown- 

 ish gi-een color. 



"In thin-section the texture is intermediate between that of a pelite and 

 that of a psammite, the pelite ground-mass being plentifully sprinkled with 

 rounded grains of feldspar. The chief constituents are orthoclase and finely 

 divided argillaceous material, including chlorite, sericite, and kaolin. There 

 are rare crystals and fragments of zircon, apatite, and magnetite. Recrystal- 

 lization has developed a mineral of hazy grayish brown to brownish green 

 color having high relief and strong bii'efringence ; it is biaxial and negative 

 with a large o])tic angle and a granular habit; it is almost certainly epidote." 



Two wells ha\e been drilled on the Silver City anticline, the places 

 l)eing indicated on the map as 1 and 2. P]ach reached the ^lississippian 

 limestone at depths a little greater than l,opO feet. In the well of loca- 

 tion 1 the drillers constantly complained that they were not finding rocks 

 with which they were familiar aiul which they had found in the wells of 

 the vicinity. When the well of location 1 proved a failure, location 2 was 

 chosen in order to get as far as possible from the altered rock of Silver 

 Citv ridae. In this well the rocks did not materially differ from those of 



