468 THE MAEQUETTE IRON-BE AEING DISTRICT. 



scattered throngli a groundmass composed of numerous microlites of feld- 

 spar in an altered basic glass. Tlie various phases of the amygdaloids are 

 so similar to one another, and so like diabasic lavas elsewhere, that they 

 demand no special description. 



THE SEDIMENT.S AND TUFFS. 



The sediments. — Petrographicallv most removed from the lavas are the sedi- 

 ments, which grade impei'ceptiblv into the tuffs. Pure sediments are found 

 only along the borders of the Clarksburg belt, beyond the horizon at which 

 the volcanic series is regarded as beginning and ending. They consist of 

 quartzites, gray wackes, and slates, many of which are much mashed. The 

 sediments interbedded with the tuffs, conglomerates, etc., are composed 

 principally of the waste of preexisting rocks, but intermingled with this 

 d(ibris is a greater or smaller quantity of basic material which is supposed 

 to be of volcanic origin. Many of the beds are now thoroughly crystalline, 

 so that an acciu-ate separation of their sedimentary and volcanic compo- 

 nents is not possible. Often some of the thicker beds consist of alternating 

 layers containing respectively large and small quantities of basic material, 

 and these pass into others in which the volcanic substance can scarcely be 

 detected. In some of the former beds a well-marked tuffaceous structure 

 is recognizable, but in the majority of cases all evidences of a well- 

 characterized original structure have been obscured by recrystalhzation. 

 As a rule the structure of the more purely sedimentary rocks is much 

 better preserved than is that of the tuffaceous ones. 



The nearly pure sedimentary rocks are dark-gray or light-gray and 

 hne-grained, with an even or a contorted bedding, marked b}^ parallel bands 

 of different shades of color. The coarser bands exhibit very plainly their 

 fragmental character on the weathered surface. Little eyes of quartz can 

 be seen against a background which has the appearance of a graywacke. 



The least altered of the sedimentary rocks consist principally of 

 rounded quartzes, altered feldspar grains, and a few flakes of a dirty, 

 greenish-brown biotite. In the finer-grained bands biotite is probably 

 more al)uudant than it is in the coarser ones, but with this exception there 

 is little difference between them. The lig-hter and darker shades noticed 



