No. 2318. PETROGRAPHY OP DIKE ROCKS OP IDAHO— SHANNON. 483 



zones crowded with fern-like microlites of hornblende and iron ore. 

 These inclusions are all somewhat rounded and embayed. (See pi. 

 39, B.) 



Mmette, Elk Creek. — In a railroad cutting just east of the mouth 

 of Elk Creek is a narrow dike of biotite-lamprophyre, cutting rocks 

 of the Prichard formation. On the south side of the cut, rounded 

 residual lumps of fairly fresh rock remain, although decomposition 

 has taken place along joints. On the north side of the cut the rock 

 is decomposed to a friable sandy aggregate containing flakes of 

 bleached and altered biotite. The dike is perhaps 4 feet wide at the 

 bottom of the cut, but narrows upward to about 8 inches, near the 

 surface. 



In appearance this rock is exactly like the rock from the Hecla 

 Mine, being composed of phenocrysts of biotite in a gray ground- 

 mass. Under the microscope the resemblance to the Hecla minette is 

 still more marked, the Elk Creek rock consisting of numerous pheno- 

 crysts of biotite with accessory iron ore and well-crystallized apatite 

 in a base of alkalic feldspar. Rarely, much-decomposed diopsides 

 occur. Abundant secondary sericite has developed from the feldspar 

 which is intensely altered. Areas of infiltrated calcite are common 

 and some secondary quartz occurs. 



That the composition given above does not make up the whole of 

 the dike at Elk Creek is evident upon examination of a specimen 

 collected here by Mr. Calkins. Although labeled the same and of the 

 same appearance in the hand specimen Mr. Calkins' specimen is 

 markedly different as seen in thin section under the microscope. 

 Here, although the sparsely disseminated biotite phenocrysts are 

 large and conspicuous they make but a small fraction of the whole 

 volume of the rock which is composed of a fine grained holocrystal- 

 line aggregate of feldspar and hornblende which is of the usual type. 

 The feldspar is so poorly individualized and so greatly altered that 

 little can be said regarding it other tlian that it is chiefly orthoclase. 

 Less abundant grains are zoned though not twinned and are prob- 

 ably plagioclase. This abrupt variation may represent an inclusion 

 or a transition toward the spessartite which occurs in the line of 

 strike on the north side of the river. Similar abrupt differences be- 

 tween different parts of the same dike are described elsewhere. Mr. 

 Calkins' specimens may be described as a porphyritic biotite voge- 

 site. (See pi. 38, A.) 



Augite-Minette., Marsh MiTie. — In the upper levels of the Marsh 

 Mine at Burke, two small dikes are exposed. On the No. 1 tunnel 

 level there is exposed one very much decomposed dike, about 6 inches 

 wide, which consists of much bleached and altered biotite pheno- 

 crysts in a pasty or sandy brownish-green matrix. This dike, some-" 

 what wider and less decomposed, is crossed by the raise between the 



