Sjpurr — Seapolite Rocks from Alaska. 311 



The above analysis bears out the results of the optical inves- 

 tigations and throws some additional light on the probable 

 variety of seapolite. The high percentage of silica in the 

 rock and the low percentage of lime indicate that the seapo- 

 lite is probably of the variety dipyre. The rock therefore pre- 

 sents some analogy to the so-called dipyre-diorite of the 

 Norwegian geologists. 



Mierocline-scapolite rock. 



This is from the same locality as the rock just described and 

 is doubtless a variation produced by differentiation. In the 

 outcrop it has the appearance of a vein, forming a distinct 

 tongue of light-colored material between walls of mottled 

 rock, which appear to be essentially the same as the seapolite 

 rock previously described, but of finer grain. This finer- 

 grained biotite-scapolite rock grades off distinctly into the 

 coarser-grained lighter-colored vein, which is about two inches 

 thick. Under the microscope the structure is very coarse and 

 the constituent grains are intergrown. Among the minerals 

 microcline prevails, with considerable seapolite, which is largely 

 altered to calcite. The seapolite gives in the basal section 

 uniform darkness and the interference cross ; it has abundant 

 irregular cleavages and cracking, and shows yellow of the first 

 order as an interference color. 



Quartz- seapolite porphyry. 

 A rock called quartz-scapolite porphyry, for want of a better 

 name, comes from a point far distant from the rocks just 

 described, being found on the Kuskokwim River at the mouth 

 of the Holiknuk, where it occurs among the light-colored 

 dikes which cut the Cretaceous shales and shaly limestones. 

 Under the microscope the rock shows phenocrysts of quartz, 

 which are of small size and are bounded chiefly by the dihexa- 

 hedral planes, but sometimes have the prism sparingly devel- 

 oped and sometimes show the faces of pyramids of two orders ; 

 the edges are often frayed by magmatic resorption. There are 

 frequent large idiomorphic phenocrysts of seapolite, more or 

 less broken down by decomposition. These phenocrysts give a 

 uniaxial figure in convergent light. The outlines of the crys- 

 tals show that they are of the tetragonal system, and they are 

 characterized by low single and double refraction ; they are 

 optically negative. There is good cleavage parallel to the two 

 lateral pinacoids, while that parallel to the prism is present but 

 not so well developed. Irregular fractures are common. The 

 mineral has decomposed along the margins and cleavages, 

 partly to calcite, but mainly to an opaque kaolinic substance 



