MAKCH24, 1899. J 



SCIENCE. 



435 



great force at a number of localities in the 

 early breccia along the east side of the range, 

 far removed from any recognized crystalline 

 body. Wherever these early breccias occur 

 dikes are apt to be a marked featui-e of the 

 country, in contradistinction to the country 

 occupied by the latter breccias. 



These dikes consist mainly of orthoclase 

 basalts, which Pi-ofessor Iddiugs, from his 

 microscopical studies, has divided into ab- 

 sarokites, shoshonites and banakites, de- 

 pending upon their varying mineral and 

 chemical composition. In the field it seems 

 impossible as yet to differentiate between 

 them, and so far as can be told they present 

 the same mode of occurrence. For most 

 geological purposes they may be grouped 

 together under the general term of absaro- 

 kites. They form a connecting link be- 

 tween many of the eruptions in the early 

 basalt sheets and the Sunlight intrusives. 

 They are closely related to the syenites and 

 monzonites of the Sunlight intrusive stock. 

 Both these dikes and sheets occur over ex- 

 tensive areas. 



Leaving for the present the Sunlight in- 

 trusives, let us take up the Ishawooa in- 

 trusives, which I select in order the more 

 easily to bring out in detail certain facts 

 bearing upon the origin of both types of in- 

 trusive rocks. Of the many intrusive 

 bodies, N"eedle Mountain, in the southern 

 end of the Absarokas, is the most imposing 

 and instructive of them all. At the base 

 runs the Shoshone Eiver, through one of 

 the most rugged and picturesque canyons 

 to be found in northern Wyoming. This 

 great stock, which stretches along the val- 

 ley for nearly four miles, rises abruptly 

 4,000 feet above the stream bed, from an 

 elevation of 7,000 feet above sea level. It 

 is overlain by 1,000 feet of partially in- 

 durated and metamorphosed breccia. From 

 the rounded summit of this commanding 

 peak the breccias may be seen stretching 

 far to the west on the opposite canyon wall, 



thence across Thoroughfare Plateau and 

 on to the higher regions of Wind Eiver 

 Plateau, where they lie nearly horizontal 

 at an elevation approximately the same as 

 that of Needle Mountain itself. Upon this 

 latter plateau the Shoshone River finds its 

 sources, and in its rapid descent of 5,000 

 feet before reaching Needle Mountain ex- 

 poses large, irregular stocks of indefinite 

 outline piercing the breccias. 



Looking eastward from Needle Moun- 

 tain, the breccias extend as far as the eye 

 can reach in the direction of the broad, 

 open plain beyond. The massive stock of 

 Needle Mountain consists essentially of 

 diorite, quartz-diorite and diorite-porx^hyry, 

 cut by numerous narrow dikes of appar- 

 ently differentiated products of the same 

 molten magma. Offshoots and apophyses 

 from the parent stock pierce the surround- 

 ing breccia, and a number of small dikes 

 penetrate the overlying breccia. From 

 these dikes sheets of granite-porphyry 

 stretch out into the breccias, and on the 

 spurs of the mountain erosion has worn 

 them bare, leaving them exposed as the 

 surface rock. The stock is found on the 

 opposite side of the canyon, rising high 

 above the stream and capped by the ever- 

 present breccia. Bordering the diorite 

 stock the breccias are indurated, crushed, 

 and so altered that not infrequently it is 

 impossible to discriminate between breccias 

 and intrusive stocks without the aid of the 

 microscope. Dr. Jaggar has shown that 

 many of these fine-grained rocks are al- 

 tered mud and silts and metamorphosed 

 breccias. 



From Needle Mountain to Mount Chit- 

 tenden, in the Yellowstone Park, a dis- 

 tance of over fifty miles, there extends in 

 a northwest direction a remarkable and 

 probably a continuous belt of intrusive 

 rocks. These intrusive bodies occur as 

 stocks, sheets, bosses and dikes, varying 

 from irregular- shaped masses of stupen- 



