PSEUDODIABASE. [)~) 



seen witli the naked eye to be a crystalline mass, though it could nirely be 

 mistaken for an ertxptive rock. It then shows dark-green bisilicates and 

 sometimes, also, feldspar grains. The absence of visible feldspar grains 

 corresponds to a microcrAstalline groundmass. Very frequently the rock 

 is fine grained, and then it sometimes retains the appearance of an altered 

 sandstone so perfectly that it is impossible to discriminate between this 

 and pseudodiabase without the aid of the microscope. 



Under the microscope it is found that at least a portion of the augite 

 exists in comparatively large grains or crystals, while the feldspar may 

 be granular or microcrystalline. In other words, as sometimes happens 

 among eruptive rocks, both granular and somewhat porphyritic forms of 

 pseudodiabase are common and are intimately associated. Under the cir- 

 cumstances a difference in chemical composition appears to be a necessary 

 inference from this difference in structure. 



Orthoclase has not been detected in the pseudodiabase. Plagioclase 

 usually forms the greater portion of the rock. In those rocks in which the 

 feldspathic mass is microcrystalline it forms a mass of minute interlocking 

 grains, sometimes resembling granophyre. These minute grains seldom ex- 

 hibit polysynthetic structure and cannot be referred to their ]n-oper species 

 by optical methods. A separation and partial analysis of a typical occur- 

 rence of this material, No. 105, Knoxville, showed that it npproaches an- 

 desine in specific gravity and composition. In the rocks in which the 

 plagioclase grains reach 0.1™" and upwai'ds in length, twin structure is 

 usual and lath-shaped individuals are common. The hemitropic lamellae 

 are often irregular, showing breaks and offsets. The extinctions refer them 

 to oligoclase or andesine. Porphyritic feldspars are uncommon, but not 

 wholly wanting. Altogether the most ordinary inclusion in the feldspar 

 consists of grains and prisms of zoisite. These are not products of the 

 decomposition of the feldspar, but of contemporaneous development, both 

 minerals being often perfectly fresh. When the pseudodiabase has not 

 been subjected to serpentinization the feldspars are often affected by a 

 decomposition process resulting in the formation of irregular grains of a 

 colorless substance wdiich o-ives orantye tints between crossed nicols. Its 



