HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 569 



(specimens 38832, 39132, and 70060), and hornblendite that of the 

 hornblende augite variety. Through hydration and other chemical 

 changes the pyroxenites give rise to serpentinous, hornbleudic, and 

 steatitic masses, as shown in specimens from Chester, Pennsylvania, and 

 Kussdorf, Saxony. The rocks appear to rauk in geological importance 

 next to the peridotites. 



The following localities are represented : 



Chester County, Pennsylvania (these rocks all more or less altered 

 into impure serpentine), 25669, 38467, 38470, 38471, 38473, 38491, 38498, 

 38484, 70160 (williamsite), and 70 LSI; near Webster, Jackson County, 

 North Carolina, 70060, 38832, 39132 ; Kussdorf and Kuhschnappel, Sax- 

 ony, 36674 and 70191, Madison County, Montana, 73175. 



B. — Effusive or volcanic rocks. 



This group includes those igneous rocks which, like the plutonics, 

 have been forced up through the overlying rocks, but which in this 

 case came nearly or quite to the surface and flowed out as lavas. They 

 therefore in many cases represent merely the upper portions of plutonic 

 rocks, from which they differ structurally, having become less per- 

 fectly crystalline, owing to their more rapid cooling and solidification. 

 The characteristic structure of the group is porphyritic, and represents 

 two distinct phases of cooling and crystallization; (1) an intratellurial 

 period marked by the crystallization of certain constituents while the 

 magma, still buried in the depth of the earth, was cooling very grad- 

 ually, and (2) an effusive period marked by the final consolidation of 

 the rock on the surface. As this final cooling was much the more rapid 

 the ultimate product is a glassy, felsitic, or sometimes holocrystalline 

 groundmass inclosing the porphyritic minerals formed during the first 

 or intratellurial stage. (See structural series and Figs. 3 and 4, PI. cxx.) 

 Those portions which have cooled wholly on the surface often show not 

 merely a vitreous form, but are vesicular or pumiceous as well from the 

 expansion of the included aqueous vapor. When the groundmass is 

 holocrystalline the rock is said to have a holocrystalline porphyritic 

 structure ; when glassy, a vitrophyric structure. An intermediate form 

 in which the groundmass is in part crystalline and in part glassy is 

 called hypocrystalline porphyritic. As would naturally be expected, the 

 rocks of this group form a series in part parallel with those of the plu- 

 tonic group, though as a matter of fact, as noted below, effusive forms 

 occur of which no plutonic equivalents have as yet been found. 



According to the geologic period of their extravasation, whether pre- 

 Tertiary or Tertiary and post-Tertiary, many authorities have found it 

 convenient to divide the rocks of this group into (1) the older or paleo- 

 volcanic effusives, and (2) the younger or neovolcanic effusives. This 

 distinction is, however, not well marked and can be of little permanent 

 value. It is nevertheless recognized to a certain extent here, inasmuch 

 as it is upheld in the leading text-book on the subject, and, moreover, 



