360 The Ainerica?i Geologist. December, i9(o 
3. Modifications due to contact with other rocks, and 
consequent absorption of foreign elements. 
The first type is characterized by a change in the texture 
of the rock, which, from granitoid, becomes very coarselv 
ophitic, and. with porphyritic variations, may change further 
to finely ophitic. and even finally to glass}' texture. This re- 
quires no change in the mineralogical or chemical composition, 
and is ordinarily accompanied by no such change. 
The second type is characterized by a marked difference in 
the relative abundance of the constituent minerals, though no 
new minerals appear. That is, chemically, as well as miner- 
alogically, the change is not qiialitative, but quantitative, 
though in the extreme types one or more of the essential min- 
erals of the normal gabbro may lack entirely. 
The third type is characterized by the appearance of nev/ 
minerals, and. often, the disappearance of old ones: thus the 
mineralogical nature of the rock may Ije wholly transformed ; 
this transformation may render the rock more acid or more 
basic ; it may make it highly aluminous, or richly ferriferrous •_ 
in short riiis chanpe is much more varied in its effects tha» 
either of the other two. and can change the composition of the 
rock in nearly any conceivable way ; and the transformation, 
both chemically and mineralogically. may be, and usually is, 
qualitative as well as quantitative. This change is often, but 
not necessarily, accompanied by a change in the texture of the 
lock, the resultant texture being often poikilitic, and usually 
fine grained. 
First type of variation. — If this kind of variation be 
considered more in detail, it is found that the coarse granular 
texture of the olivine gabbro (see plate VIII, figurei) changes 
by insensible degrees (see plate VIII, figure 2) until it becomes 
clearly ophitic, (plate IX, figure 2) without any change in the 
mineralogical composition, and with no appreciable changes in 
the chemical composition. Similar series exist from the oliv- 
ine-fiee gabbro to the diabase containing no olivine, and could 
probably be found between the granular and ophitic represen- 
tatives of the other varieties. Nearly all observers agree that 
these variations are equally imperceptible and gradual in the 
field. This change in texture produces very little change in 
external appearance, and is detected only with difficulty unless 
