THE CLAKKSBURd FORMATION. 471 



in contijj^uons hiiiids iirc due niaiiih' to (liticreiices in tlic (|u;uitity of 

 magnetite, biotite, iuid certain indefinite dust particles present. In all these 

 rocks the plagioclase is altered, and among- its alteration jiroducts are 

 found biotite, quartz, and occasionally a little muscovite. 



The greater number of the sedimentary beds are characterized l)y the 

 presence of amphibole. They comprise rocks whose difference in degree 

 of crystallization is dependent apparently upon the proportion of aniphilxde 

 present in them. Tliis amphibole is certainly not a product of the decom- 

 position of the usual constituents of a sedimentary rock; on the otluir 

 hand, it is the most common product of the alteration of the igneous rocks 

 associated with the fragmental ones. For tliis reason largely, and because 

 certain rocks interleaved with the sediments are composed almost exclusively 

 of amphibole, while at the same time they are un(juestionably marked by 

 bedding lines, and further, because the hornljlende rocks, by the gradual 

 loss of iheir hornblende, pass into the sediments, the material from which 

 the hornblende in the sediments was formed is believed to have l)een 

 tuffaceous. 



On the weathered surface the majority of the hornblendic sediments 

 present a very rough aspect, a consequence of the projection of the horn- 

 blende crystals beyond the general surface of weathering. The rocks are 

 dark-green on a fresh fracture, where they look like massive crystallines. 



The thin sections show a groundmass which surrounds the hornblendes. 

 The amphilxile itself is in large, cellular, green jdates, filled with inclusions 

 of the rock's conq)onents. In cross-section many of the i)lates have the 

 idiomorphic outlines of crystals, but in longitudinal section they are more 

 or less irregularly shaped (see PI. XXXII, fig. 1), the ends especially 

 frayino- out into long needles which penetrate the groundmass. 



The groundmass is composed of ([uartz, altered plagioclase, biotite, 

 chlorite, and magnetite, with, sometimes, light-colored garnets in little 

 dodecahedra. Where the rock is massive the garnets are less plentiful 

 than they are in the scliistose ])hases. 



This groundmass has a coarse and a fine part, the latter seiwing as a 

 matrix to the former. The coarser part is composed of quartz and biotite. 

 Some of the quartz grains show the rock to be fragmental, since a few 



