WHITE ROCK ZONE OF SQUARE RUTTE. 405 



the other, they were not formed by two separate intrusions, but that, on 

 the contrary, tliey are a geologic unit, and that tlie mass as a whole was 

 intruded at one and the same time and cooled and crystallized under 

 the same conditions, and that tlie exi)lanati()n of the peculiarities which 

 it presents must he sought in another way — one which has an important 

 hearing on theoretic ])etrology. 



As one approaches nearer the top no more black rock is seen ; the re- 

 mainder of the mass is of pure white or pinkish syenite and it presents 

 everywhere the same even grain. The same platy structure continues 

 and at times there are no talus slopes, vegetation, lierbage, or even soil, 

 onl}* vast smooth white surfaces of naked rock on whose almost polished 

 slopes it is impossible to climb. Towards the top the average thickness 

 of the plates increases somewhat and their dip gradually becomes less 

 until eventually they are horizontal. It is b}^ their breaking off when 

 horizontal that the enormous ring-shaped, mural precipice which forms 

 the top has l)een made. The regularity of this platy jointing, together 

 with the even rounding of the corners through weathering where the 

 joint planes cross, gives a most remarkable. likeness to colossal masonry 

 in the upper walls. The mural front seen from the plains below has so 

 close a resemblance to l)edded and jointed sedimentary strata that only 

 close inspection shows it is not. On tlie top the slabs produced by joint- 

 ing are often of great size, forming large tables of stone about 2 feet thick 

 by 15 or 20 feet long and half as In'oad. 



RIG IX OF THE PLATY PARTIXG, 



From wliat has been already said in regard to the jdat}' i)arting which 

 forms so marked a feature of Square butte, it will be seen that it bears the 

 same relation to the mass as a whole as do the enfolding leaves of an 

 onion to the bulb cut in half by a horizontal plane. 



We V)elieve that they therefore represent parting surfaces ]iarallel to 

 the former covering of tlie laccolite from which the isothermal planes of 

 cooling descended into the mass. We can conceive of no other h3'poth- 

 esis which would give a reasonable exi)lanation of their arrangement 

 and disi)osition, and since S([uarc butte is unquestionably an intrusive 

 ma.ss we regard them as one of the strongest proofs of its laccolitic 

 nature. 



Such an arrangement of the parting i)laiU'S of a cooling igneous mass 

 is by no means unknown, however, as it fre(|uently occurs in the great 

 plionolite domes of central Europe. 



Tin-: WHITE UAXD. 



Tliere still remains an interesting feature of tin.' butte to Ix; described. 

 This is the presence on the south side of a band of white rock which 



LVIII- Bir.i.. Geoi.. Soc. Am., Voi, C, 1804. 



