754 RKroni— 1893. 



ty a period of extensive erosion, reducing the country to nearly its present form. 

 Then came the eruption of a vast flood of rhyolite constituting the Parlv plateau, 

 which was followed in this region by smaller outbreaks of basalt. The last phase 

 of volcanic activity is found in the geysers and fumaroles which have rendered the 

 region famous. 



The volcano of Crandall Basin consists chiefly of the flrst series of basic ande- 

 sites and basalts. The earliest acid andesite which occurs beneath these rocka 

 appears to be the remnant of eruptions from neighbouring centres. 



Nothing remains of the original outline of the volcano. The district is now 

 covered by systems of valleys and ridges of mountain peaks that rise 2,000 to 5,000 

 feet above the valley bottoms. The geological structure of the country, however^ 

 makes its original character evident. 



The outlying portions of the district to the south, west, and north consist of 

 nearly horizontally bedded tuff's and subaerial breccias of basic andesite and basalt. 

 With these are intercalated some massive lava flows, which are scarce in the lower 

 parts of the breccia, but predominate in the highest parts above an altitude of 

 10,000 feet. Here they constitute the summits of the highest peaks. 



In contrast to the well-bedded breccias around the margin of the district the 

 central portion consists of chaotic and orderless accumulations of scoriaceous 

 breccia with some massive flows. These breccias carry larger fragments of rock and 

 exhibit greater uniformity in petrographical character. 



A still more noticeable feature of the central portion of the district is the 

 occurrence of dikes, which form prominent walls, and may be traced for long dis- 

 tances across the country. 



The greater number of them are found to converge toward a centre in the highest 

 ridge ia the middle of the drainage basin of Orandall Creek. A small number 

 converge towards a second centre three or four miles east of the first. In the 

 southern part of the district there are many dikes trending towards a centre near 

 the bead of Sunlight Basin, about fifteen miles south of the Crandall centre. 



The centre towards which the Crandall dikes converge is a large body of 

 granular gabbro graduating into diorite. It is about a mile wide, and consists of 

 numerous intrusions penetrating one another, and extending out into the sur- 

 rounding breccia, which is highly indurated and metamorphosed in the immediate 

 vicinity of the core. AVithin the area of indurated breccia the dike rocks become 

 rapidly coarser-grained as they approach the gabbro core. This was undoubtedly 

 the central conduit of an ancient volcano, the upper portion of which has been 

 eroded away. 



Upon comparing the geological structure of this region with that of an active 

 volcano like Etna it is apparent that the lava flows which form the summits of the 

 outlying peaks musthave been derived from lateral cones fed by dikes radiating from 

 the central conduit ; and assuming that tlie volcano of Crandall Basin was similar in 

 type to that of Etna, an idea of its original proportions is derived by constructing 

 upon profile sections through the Crandall core the outline of Etna. If the erosion 

 of the summits of the highest peaks is neglected the resulting height of the ancient 

 volcano above the limestone floor is estimated at 13,400 feet. This is undoubtedly 

 too low, and is well within the limits of present active volcanoes. 



Erosion has removed at least 10,000 feet from the summit of the mountain to 

 the top of the high central ridge in which the granular core is situated, and has 

 cut 4,000 feet deeper into the valleys on either side. It has prepared for study a 

 dissected volcano, which, it is hoped, will in time reveal some of the obscurer 

 relationships existing between various phases of igneous rocks. 



3. On Structures in Eruptive Bosses wliicTi resemble those of ancient 

 Gneisses. By Sir Archibald Geikie, F.B.S. 



While it is now the general belief of geologists that the older granitoid and 

 banded gneisses were originally eruptive masses, considerable difference of opinion 

 exists as to the cause of the peculiar and characteristic structure which distin- 

 guishes gneiss from ordinary amorphous eruptive material. The pregnant sugges- 



