PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 427 



Amygdaloid. Chlorastrolite.] 



These areas are not very sharply separated from the epidote, and it seems possible 

 that the epidote has replaced the mass of the rock. However, what at first glance 

 appears like altered augite or glass is in reality largely native copper, and the 

 apparent feldspars are composed of fibres of thomsonite. A little quartz is present in 

 the section. 



One section. 



Cabotian(?) 



The section apparently shows a basic rock replaced by epidote, 

 copper and thomsonite, although the evidence from this one section is not sufficient 

 to demonstrate this fact. u. s. G. 



No. 570. AMYGDALOID. 



From the rock at the light-house at the entrance to Rock harbor. 

 Rff. Annual Report, x, page 53. 



Mi. </. A brown amygdaloidal trap showing calcite, laumoutite, chlorastrolite 

 and chlorite; also apparently a little thomsonite. 



Mic. The augife and feldspar microliths are ophitic. The olivine is almost 

 wholly converted to /><r/ht</ifc, which is partly brown and partly almost isotropic, in 

 the latter case approaching t halite, of which there is a considerable quantity, some 

 of it being in spherulitic masses. 



Two sections. 



Age. Cabotian(?) N. H. w. 



No. 570A. CHLORASTROLITE. 



From the beach at Rock harbor, Isle Royale. These are weathered from the trap of the region and accu- 

 mulate with the gravel on the beach. Compare No. 566. 



Rcf. Annual Report, x, page 53; American Geologist, vol. xxiii, page 116. 



JA/c. When this zeolite has its perfect development, without impurities and 

 without inclusions, the exterior surface of the isolated pebbles presents a stellated 

 and radiated coloration in green of two shades. Blocks of dark green, having a 

 polygonal or irregular periphery, are separated from each other and surrounded by 

 narrow bands or lines of a light green, the two shades blending rapidly at the contacts. 

 Thus the pebbles, which have a sufficient hardness, possess a rich, flecked network of 

 green, and a beauty which has caused their adoption as an ornamental gem. They 

 vary in size from about half an inch downward to mere specks. 



The specimens, however, which have this character in perfection, are not abun- 

 dant. Many of the pebbles are not beautifully marked, but have a dull green color in 

 general, and doubtless should not be allowed the name of chlorastrolite. This 

 general greenness is sometimes quite dark, and in other cases it seems to fade into a 

 very light green, resembling lintonite, in the same way that mesolite fades into 

 lintonite. Outwardly this light green substance is structureless, quite hard and 



