120 UNIVERSITY OF COLORADO STUDIES 



SUGARLOAF DlKE 1 



Since the dike passing through Sugarloaf Mountain is the most 

 prominent and least altered, and presents a fairly wide range in mineral 

 composition, the rock will be described in some detail, and this may be 

 taken as a fair average of the latite in general. On the mountain the 

 latite is a brownish-gray aphanitic rock containing megascopic crystals 

 of feldspar, biotite, pyroxene, hornblende and less abundant titanite. 

 Of these the feldspar phenocrysts are the most numerous, usually less 

 than 2 mm. in diameter. They are white, sometimes glassy, and occa- 

 sionally show striae under a hand lens. The shiny biotite flakes are 

 commonly less than i mm. in diameter. It is difficult, if not impossible, 

 to distinguish between the pyroxene and hornblende in the hand speci- 

 men. Sometimes crystals of these minerals reach a length of 10 mm., 

 but are for the most part under 3 mm. in length with a thickness of less 

 than 1 mm. Segregations of one or both of these minerals, with which 

 are mingled small grains of feldspar, are not uncommon in masses of 

 several cubic centimeters. Yellow titanite in small crystals with a high 

 luster is a constant accessory. At the top of the mountain the dike is 

 much jointed, prismatic blocks being the prevailing type. Four-sided 

 and five-sided columns are the commonest, but hexagonal forms are 

 not rare. The columns are often long in proportion to the thickness; 

 occasional blocks but 4 to 6 inches thick are 4 feet in length. 



Under the microscope the rock is more distinctly porphyritic with 

 phenocrysts of plagioclase, orthoclase, biotite, augite and hornblende. 

 The feldspars are commonly tabular in habit, but sometimes approach 

 equidimensional forms. The ratio of plagioclase to orthoclase is approxi- 

 mately 2 to 1. In the plagioclase albite twinning is frequently accom- 

 panied by pericline or Carlsbad twinning. The maximum extinction 

 angle in sections normal to the albite lamellae of 25 indicates that the 

 feldspars are probably not more basic than labradorite with a compo- 

 sition of about Abx An x . Zonal banding is often present, though not 

 pronounced, and different zones are not markedly different in optical 

 properties. Alteration products are calcite, quartz and kaolin. Ortho- 



1 A very careful study of rock taken from the peak was made several years ago by Mr. Barry Hogarty 

 and the results published under the title of "The Andesite of Mount Sugarloaf" in the Proceedings of the 

 Colorado Scientific Society, Vol. VI, pp. 173-85- 



