REPORT OF THE CHIEF ASTRONOMER 367 



SESSIONAL PAPER No. 25a 



to the range of extinctions in an orthoclase crystal elongated parallel to the a 

 axis. The birefringence is low and is like that of orthoclase in very thin 

 sections. In fact, it seems highly probable that most of the material which 

 has replaced the original phenocrysts, is orthoclase. This conclusion is upheld 

 by the study of the chemical analysis of the rock. 



The spherulitic substance is regularly mixed with a small amount of 

 obscurely granular material, showing characters like those of hydronephelite, 

 and with other, similarly obscure, pale greenish-gray substance in minuie leaf- 

 aggregates which have the optical properties of serpentine. A zeolite, like 

 stilbite or desmine, may also be present. All of these materials form a matrix 

 in which very small microlites of avigite, biotite, magnetite, and apatite — 

 inclusions in the original mineral — are embedded. 



To the writer the best interpretation of these round bodies is that they 

 represent pseudomorphs after phenocrystic leucite; their optical resemblance 

 to the described pseudoleucites is certainly great. The alteration of the leucite 

 seems to have taken place as a kind of magmatic after-action, rather than as 

 the result of ordinary weathering, for the ferromagnesian minerals are ideally 

 fresh. 



The ground-mass in which the large pseudoleucites and the other idiomor- 

 phic minerals lie, is generally quite like that of the pseudomorphs except that 

 there are no outlines even remotely suggesting the crystal form of leucite. 

 Neither here nor in the phenocrystic bodies is there any certainly isotropic 

 material, nor any structure which could have been inherited from the twinning 

 bands of leucite. Nevertheless, the similarity of ground-mass and phenocryst 

 indicates that they were originally composed of the same material, chemically 

 if not mineralogically. The simplest assumption is that the ground-mass of 

 the rock was chiefly allotriomorphic leucite, which, like the leucite of the pheno- 

 crysts, was unstable during the cooling period following crystallization. 



This view, cannot, with the material in hand, be proved, but it is strongly 

 upheld by the close chemical parallel between this rock and the typical missour- 

 ite described and named by Pirsson.§ In that species the constituent minerals 

 are apatite, iron ore, olivine, augite, biotite, leucite, and some zeolitic products. 

 The leucite is there unquestionably present and is interstitial. The obvious 

 differences between the Record mountain dike (as originally crystallized) and 

 the type missourite consist in the presence of about five per cent of phenocrystic 

 leucite and the absence of olivine in the British Columbia dike. The presence 

 of olivine in a rock of this kind is not a matter of principal importance, for, 

 as Pirsson has pointed out, biotite may be considered as the chemical equival- 

 ent of a mixture of leucite and olivine. 



In Table XXV the result of Mr. Connor's analysis of the dike (specimen 

 ]\ o. 541, Col. 1 ; molecular proportions in Col. la) and the analysis of the type 

 missourite from the Highwood mountains, Montana, (Col. 2), are given. 



§ L. V. Pirsson, Bull. 237 U.S. Geol. Survey, 1905, p. 115. 



