Pogue, Jr. — Geology and Structure of Volcanic Rocks. 229 



in width and 3 to 4: miles in length to ones indicated only by 

 the presence of a few bowlders. The rock shows upon the 

 surface as rounded, yellowish bowlders, ranging in size up to 

 10 feet in diameter, . and distributed in lines following the 

 trend of the dikes. 



The directions of the dikes in all cases coincide with the 

 schistosity of the formations in which they occur. Also, the 

 gabbro is itself unmashecl. The schistosity is therefore con- 

 sidered to have been developed prior to the introduction of 

 the dikes and to have been, as an easy line of yielding, a con- 

 trolling factor in their entrance. 



The contacts between dikes and 'adjacent formations are 

 much obscured by weathering, so that it is impossible to 

 discern any contact effects. The deeply weathered nature of 

 the contacts, however, bespeaks a zone susceptible to altera- 

 tion and doubtless rendered so by contact action. Jointing is 

 well developed, and of such a nature as to suggest the opera- 

 tion of a compressive force after the introduction of the dikes. 



The gabbro is a greenish-gray rock, of medium grain and 

 homogeneous texture, in which crystals of green hornblende 

 and areas of opaque feldspars may be recognized. The micro- 

 scope reveals that the hornblende is uralite, secondary after 

 pyroxene, and that the feldspar has been completely changed 

 into saussurite, which represents an original plagioclase rich 

 in lime. The rock is tough and heavy, and is very susceptible 

 to weathering. In point of age, it is the second youngest 

 rock in the district, since it cuts the other formations and is 

 itself cut by dikes of diabase. 



Diabase. — The diabase forms narrow dikes uniformly, 

 though not abundantly, distributed throughout the district. 

 It shows upon the surface as narrow lines of small rounded 

 bowlders of an iron-rust color, locally called "niggerheads". 

 The dikes vary in size from a few feet in width and a few yards 

 in length to the largest, which is about 100 feet in width and 

 slightly over a mile in length. The majority conform to the 

 former dimensions. In trend they usually vary from JST. 30° W. 

 to N. 30° E. The trends of the dikes coincide with important 

 joint directions. 



The diabase is a massive, fine-grained, dark blue rock, very 

 tough and with a waxy luster on fresh fracture. Upon exam- 

 ination it is seen to be a closely knit aggregate of dark-colored 

 minerals, among which striated feldspars may be distinguished 

 from the ferromagnesian minerals. Although the rock is high 

 in olivine, this constituent cannot be megascopically distin- 

 guished from the augite. The mineral composition, or mode, 

 determined by the Rosiwal method,* gave : 15*6 per cent 



*Kosiwal, Verb. Wien. Geol. Keichsanst., vol. xxxii, p. 143 ff., 1898. Cf. 

 Cross, Iddings, Pirsson, Washington, The Quantitative Classification of 

 Igneous Eocks, p. 204. 



Am. Jour. Sci. — Fourth Series, Vol. XXVIII, No. 165. — September, 1909. 

 16 



