REPORT OF THE CHIEF ASTRONOMER 401 



SESSIONAL PAPER No. 25a 



Rock Creek Chonolith. 

 structural relations. 



Just above its confluence with Kettle river, Rock creek flows through a 

 narrow steep-walled gorge in which excellent sections, not only of the Oligo- 

 cene sandstones and conglomerates but also of an intrusive porphyry cutting 

 the sediments, may be studied. For nearly two miles the creek flows between 

 walls composed of the porphyry, or of the porphyry overlain by the sandstone- 

 conglomerate formation. A somewhat prolonged field study of this mass of 

 porphyry showed that it covers about five square miles within the five-mile 

 Boundary belt. It is the largest known body of this rock in the region. 



At several points the porphyry splits the Tertiary elastics after the manner 

 of a thick sill or laccolith. At other points the intruded mass followed the con- 

 tact-surface of the unconformable Tertiary and Paleozoic rocks. At still other 

 points the intrusive contact cuts across Tertiary conglomerate and Paleozoic 

 quartzite or schist alike. For long distances the contact-line of the porphyry 

 is, as shown on the map, remarkably straight but is characterized by 

 several rectangular bends. These elements of form in the contact-surface are 

 such as might be produced if the magma has entered an opening bounded by 

 two systems of faults or master joints cutting each other at right angles. The 

 intrusive is never equigranular and nowhere displays the characteristics of 

 a plutonic rock of the batholithic order. The steady persistence of the porphy- 

 ritic structure even in ledges farthest removed from contacts is, on the other 

 hand, a good indication that the whole mass has been injected and does not 

 enlarge downwardly, like ii stock or batholith. The exceedingly irregular form 

 of the body and its relation to the invaded formations forbids our classifying 

 it among the laccoliths. It has, indeed, been taken as a type of the intrusive 

 bodies called ' chonoliths ' by the writer.* 



The porphyry has two strongly contrasted phases. One of these character- 

 izes the principal part of the chonolith; the other is regularly found along 

 its walls and roof and is the product of rapid chilling on the contacts. 



DOMINANT ROCK TYPE. 



General Description. — The principal phase is a greenish-gray, nearly or 

 quite holocrystalline, fine-grained rock which is often so abundantly charged 

 with phenocrysts as to appear equigranular. The microscope shows, however, 

 that the porphyritic structure is always present. The phenocrysts always include 

 rhomb-shaped feldspar, and augite,, which are generally accompanied by 

 biotite and a few small olivine crystals. In some cases biotite is a more 

 important phenocryst than augite. The feldspar rhombs vary from 2 mm. to 

 5 mm. in length ; the augite prisms, from 1 mm. to 3 mm. in length ; the biotite 

 foils, from 1 mm. to 4 mm. in diameter. The olivines are seldom clearly visible 

 to the naked eye, partly because of their being serpentinized and partly because 



•Chap. XXV.; also Jour. Geology, Vol. 13, 1905, p. 499. 

 25a— vol. ii— 26 



