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[Vol. 3. 



ular boundaries between the two minerals apparently indicate 

 that they mutually interfered in crystallizing. 



The structure of the groundmass is typically intersertal. 

 Between the plagioelase laths lie rounded granules of augite, 

 crystals of magnetite, and a very small amount of brown glass. 

 There are a certain proportion of laths distinguished by their 

 greater size, and these are characteristically interspersed with 

 augite inclusions generally in a row in the centre of the crystal. 



The minerals are all perfectly fresh and there is no secondary 

 material to indicate the prior presence of olivine. The rock, how- 

 ever, is classed as a basalt because of the basicity of the feldspars 

 and the important part played by the augite. 



ROCKS OP THE MASCALL FORMATION. 



General Classification . — The Mascall Formation near its base 

 contains beds probably of organic origin, containing the imprints 

 of leaves. At one locality the section is partly made up of well 

 rounded gravels. But the dominant material resembles the John 

 Day tuffs in texture, in general mode of weathering, and in part 

 in the heavy character of the bedding, though fine lamination 

 and even crossbedding are not uncommon. The material after 

 laboratory examination is believed to be in greater part of 

 pyroclast ic origin . 



Canon City Leaf Beds. — The matrix of the leaves at this 

 locality is a white, fine-grained chalky substance, so light that it 

 floats on water. Examined under the microscope, the powder, 

 although it consists largely of fine particles of indeterminate char- 

 acter, contains some bodies that are apparently of organic nature. 

 These include slender rods and, more characteristically, little 

 tubes of circular cross-section, with transverse partitions like 

 stems of bamboo. 



Tuffs. — The material in which the fossils are imbedded is 

 usually light-colored, fine-grained and of harsh texture. It is 

 not sufficiently coherent to be cut into macroscopical sections. 

 The rock powder is found to consist in large part of angular 

 glass fragments, with much indeterminate opaque dust and a few 

 glassy and angular grains of feldspar. There is no doubt that 

 these rocks are acid tuffs. 



