VOLCANIC ROCKS 253 



is usually in the form of needles, and there is sometimes pyroxene 

 present, as well as biotite and ilmenite. 



Diorite dikes are very abundant in the southern flanks of the Silver 

 Peak range/ both in the Ordovician sediments and in the large areas of 

 quartz-monzonite. Many of these dikes are indicated on the geological 

 map in Spurr's report. They appear to be normal diorites. The amphi- 

 bole is often in the form of needles, and the feldspar in the form of laths. 

 There is biotite present and secondary epidote. There are some diorite 

 dikes in the Palmetto mountains, and very abundant diorite dikes in the 

 slates and limestones 5 miles west of south from Piper peak. 



Volcanic Eocks 

 age 



The lavas of the Silver Peak quadrangle may be divided into an older 

 series, associated with Paleozoic rocks and probably Paleozoic in age, and 

 a later series, of Tertiary age. The Paleozoic lavas are usually much 

 altered, which condition is expressed by the prefix meta. Thus there are 

 fresh rhyolites in the Tertiary and meta-rhyolites in the Paleozoic. 



META-RHYOLITE 



Ehyolitic lavas which have undergone alteration are designated meta- 

 rhyolite. In such lavas the original glassy groundmass has become more 

 or less crystalline. Such a groundmass originally glassy is sometimes 

 called a devitrified groundmass. The devitrified dacites or meta-dacites 

 are here placed with the meta-rhyolites. All of the rocks grouped under 

 the head of meta-rhyolite are presumed to be pre-Tertiary in age, and 

 most of them are known to be of Ordovician or Cambrian age. The meta- 

 rhyolites, so far as known, are the oldest lavas of the Silver Peak 

 quadrangle. They form bands or lenses in Paleozoic sediments most 

 abundantly in the Palmetto mountains, but are found in all parts of the 

 quadrangle. No analyses were made of the lavas except from dikes. 

 Many of these dikes are completely crystalline, and analyses of three of 

 them indicate a high content of soda. The examination of the thin- 

 sections of the dike rocks shows that some of them contain little or no 

 quartz, as, for example, numbers 319 and 343 of the table of analyses. 

 Soda-rich, completely crystalline igneous rocks may be called micro-soda- 

 syenite or keratophyre. Number 319 evidently has the composition of 

 an alkali-rich andesite or latite and number 343 of a quartz-keratophyre 

 or soda-rhyolite. Other dikes are fine grained and evenly granular, and 



