MUSEUM OF COMPARATIVE ZOOLOGY. 279 



magma, which has torn and eaten its way into the olivine substance. 

 Occasionally we see only little heaps of black grains left from the de- 

 struction of the olivine, every gradation being traced from these to the 

 only slightly attacked olivine. Similar grains are scattered through the 

 "•roundmass, giving rise to the belief that part of the magnetite found 

 in the basalt was derived in this way, as the crystals of olivine are often 

 found to be magnetic. The augite, unlike the olivine but like the 

 feldspar, is a direct product of crystallization from the cooling magma. 

 It is found to vary inversely with the olivine, and to crystallize with 

 that mineral in such a manner as to lead to the conclusion that the 

 augite is derived from the crystallization of the olivine material mixed 

 with a certain proportion of the base. The olivine is very rarely the 

 product of crystallization of the magma, but belongs to the minerals of 

 the first class. The more coarsely crystalline of the basalts are known 

 as dolerites and anamesites ; the same general characters also hold 

 good in the leucite and nephelite varieties. 



The basaltic rocks are more subject to alteration than any others, the 

 variety of alteration products diminishing as the silica increases. This 

 alteration affects the olivine and base first, the latter often becoming 

 somewhat fibrous, forming a base called by the German lithologists 

 " micro-felsitic." Under this term they have utterly confounded the 

 original niicro-felsitic base of the unaltered andesites, and of some allied 

 rocks, with the alteration and devitrification products in the basalts, 

 andesites, trachytes, rhyolites, felsites, etc. Further alteration obliter- 

 ates the base, and, one after another, the minerals in the rock, the feld- 

 spar usually being the last to disappear. These changes give rise to 

 the presence of hydrous oxides and silicates, carbonates, etc. The chief 

 proportion of quartz in these altered rocks is a direct product of altera- 

 tion, derived from the base and the silicates in the basalt. The fine- 

 grained, compact basalts through their alteration give rise to the variety 

 known as melaphyr, while the coarser-grained ones, like the dolerites, 

 in like manner form the diabases and the great majority of the basic 

 rocks known as diorite, — a name that is at present the most abused one 

 that occurs in litholog}% as it is made to cover, not only old basalts, 

 andesites, etc., but also the granites of the Sierra Nevada. May it soon 

 be dropped! The still more coarsely crystalline varietie's, when altered, 

 give rise to the gabbros and peridotites. Under the former of these 

 should be placed the majority of the eupliotides, in which the saussurite 

 is an altered feldspar. For the special descriptions and the tracing out 

 of these changes, reference must be made to the original paper. The 



