408 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



perfected. Meantime, in naming the rock rhomb-porphyry the need of the field 

 geologist is the better satisfied. It may thus be defined as a basic rhomb-por- 

 phyry rich in augite and biotite; where vitrophyric, it is charged with much 

 original water resident in the more or less zeolitized glass. 



CONTACT PHASE OF THE CHONOLITH. 



For distances tinder fifty feet, measured perpendicularly from the contact, 

 the chonolithic rock has a field-habit notably unlike that of the phase just 

 described. This difference is due simply to chilling on roof or walls. Within 

 this chilled zone the rock is a dark bluish-gray or slaty-gray, very fine-grained 

 porphyry. The phenocrysts are again chiefly rhomb-feldspars which are here 

 quite glassy and less charged with inclusions and secondary calcite than the 

 feldspar of the central phase. They measure from 2 mm. or less, to 6 mm. in 

 length. With them, a few augite prisms up to 2 mm. or 3 mm. long, and 

 hexagonal bio'tite foils from 1 mm. to 2 mm. in diameter, may be seen in the 

 hand-specimen. At other localities biotite is not phenocrystic but is abundant 

 in the ground-mass. 



A typical specimen of this phase will serve as a basis for its further des- 

 cription. It was collected at the same group of ledges from which the analyzed 

 specimen of the central phase was taken but at a point only ten feet from the 

 invaded conglomerate. Under the hammer the chilled phase breaks with a 

 sonorous, almost metallic ring which, of itself, indicates the signal freshness 

 of the rock at the average outcrop. 



Under the microscope this type specimen of the chilled phase shows a few 

 small serpentinized olivines among the more conspicuous phenocrysts. The acces- 

 sories are the same as in the central phase with the exception that nephelite 

 was nowhere recognized in the thin section. The ground-mass is hyalopilitic 

 with many minute, acicular augites, thin biotite foils, and feldspar microlites, 

 embedded in a base which is almost certainly a true glass somewhat zeolitized. 

 This base, composing at least 40 per cent of the rock by volume, generally 

 polarizes with exceeding faintness, the anisotropic property being only appre- 

 ciated with the gypsum plate. Just how far the anisotropy is due to devitrifi- 

 cation and how far, possibly, to the straining of the glass, cannot be declared. 

 In spite of close study, with high magnification, there is no certain clue as 

 to the nature of the devitrification product or products. That a large amount 

 of glass still remains is indicated by the low specific gravity of the specimen, 

 viz., 2-608. Other fresh hand-specimens have specific gravities of 2-564 and 

 2-645. The average specific gravity of this phase is about 2-600, and contrasts 

 with that of the central phase, which is about 2-740. Excepting the small 

 amount of serpentine derived from the olivines, here and there a minute point 

 of calcite, and the undetermined zeolite of the base, there are no noteworthy 

 secondary products visible in thin section. The rock is exceptionally fresh. 



Professor Dittrich's analysis of this specimen (No. 1053) gave the following 

 result : — 



