256 PROCEEDINaS OF THE AMERICAN ACADEMY 



The fine-grained diabases are dark-brown, or gray, and green, with 

 subconcboidal to irregular fracture, generally tough, but not hard, on 

 account of the relative abundance of chlorite. Under the microscope, 

 they are found to consist exclusively of triclinic feldspar, — sometimes 

 associated with orthoclase, — and of augite and magnetite, and the 

 alteration products of these minerals. They contain no chrysolite, 

 nor any remnants of the fluid magma in the interstices between the 

 constituents. The feldspar crystallized first, and the interstices were 

 occupied by the augite and magnetite. The feldspar crystals are gen- 

 erally in square prisms, from a lengthening of the zone : it. 



The feldspar appears, from measurements, of the angle between the 

 principal sections in alternating hemitropic bands by Des Cloiseaux's 

 method, to be oligoclase in some instances, in some auorthite, and in 

 others labradorite. 



In some of the fine-grained varieties which occur more rarely, there 

 is very little augite, and the rock has a light-gray color ; this is notably 

 the case with a great thickness of rocks on the Hungarian location, and 

 to a much less degree with beds 45 and 66 of the Eagle River Section. 



The coarse-grained varieties, with marked differences of external 

 appearance as regards color and grain, have the same characters under 

 the microscope as the finer grained, except that they contain much 

 more numerous crystals of apatite, and that they often differ in regard 

 to the secondary products. 



The melaphyres are sharply marked. The constituents are plagi- 

 oclase, augite, magnetite, chrysolite, and remnants of the fluid magma, 

 occupying, in more or less small quantity, the intei'stices between the 

 constituents. The chrysolite is generally more or less altered, and 

 the remnants of the fluid magma are only rarely preserved in the form 

 of glass base, for instance, in bed 90, Eagle River Section ; it is almost 

 always altered to a green chloritic substance. Beds Nos, 108 — " the 

 Greenstone," — 90, and 04, all in the Eagle River Section, are types 

 of the melaphyre, and to it belong, generally, what are locally known 

 as mottled traps. 



In the melaphyre, the feldspar appears to be generally anorthite, 

 to judge from the size of the angles between the principal sections 

 in alternating hemitropic bands in sections cut at random in the 

 zone : it. It is in sharply defined, long, narrow crystals, enclosed 

 in the younger augite. 



The melaphyre is less subject to the changes that produce the in- 

 termediate or amygdaloidal form, and is not so apt to have a true 

 amygdaloid for the top of the bed. 



